regmap: fix of_regmap_get_endian()

Commit d647c199510c ("regmap: add DT endianness binding support") has
some issues. Specifically, if config->reg_format_endian is not explicitly
set, it will be zero, i.e. REGMAP_ENDIAN_DEFAULT. The switch statement
that looks up the *endian from DT for the type==REGMAP_ENDIAN_VAL case
doesn't change *endian in the type==REGMAP_ENDIAN_REG case. However, the
test immediately following, compares *endian against REGMAP_ENDIAN_NATIVE,
and if not equal, returns *endian as is. This ends up returning
REGMAP_ENDIAN_DEFAULT, which the calling code does not expect, eventually
leading to e.g.:

Unable to handle kernel NULL pointer dereference at virtual address 00000024
...
[<c02efd64>] (regcache_cache_only) from [<c0465fa8>] (tegra30_ahub_probe+0x1b8/0x430)
[<c0465fa8>] (tegra30_ahub_probe) from [<c02e196c>] (platform_drv_probe+0x2c/0x5c)
[<c02e196c>] (platform_drv_probe) from [<c02e0580>] (driver_probe_device+0x10c/0x22c)
[<c02e0580>] (driver_probe_device) from [<c02e072c>] (__driver_attach+0x8c/0x90)

This patch solves this by:
* When looking up the endianness from DT, don't change *endian at all if
  there is no DT property; leave it set to REGMAP_ENDIAN_DEFAULT so the
  code falls through to other data sources in the same way as before.
  Now, the "unspecified" case acts the same for both REGMAP_ENDIAN_REG and
  REGMAP_ENDIAN_VAL.
* After potentially looking up the endianness from DT, check *endian
  against REGMAP_ENDIAN_DEFAULT instead of REGMAP_ENDIAN_NATIVE to avoid
  returning unexpected values.

Also, clean up the code a bit:

* Make the comments briefer, and only refer to the specific action taken
  at their location. This makes most of the comments independent of DT,
  and easier to follow.
* Restore the overall default of REGMAP_ENDIAN_BIG if none of the config,
  DT, or the bus specify any endianness. Since all busses specify an
  endianness now, this makes no difference, but I saw no justification in
  the patch description for changing the default default.
* s/of_regmap_get_endian/regmap_get_endian/ since the function isn't DT-
  specific, even if the reason it was originally added was to add some
  DT-specific features.

Cc: Xiubo Li <Li.Xiubo@freescale.com>
Fixes: d647c199510c ("regmap: add DT endianness binding support")
Signed-off-by: Stephen Warren <swarren@nvidia.com>