Fix unlocking function for most Atmel AT2[56]D* chips.

I broke unlocking them correctly in r1635 while refactoring (NB: the
commit log including the overly selfconfident statement about the
"bug in spi_disable_blockprotect_at25df()").

Affected chips have per sector protection bits and the write protection bits
in the status register do indicate if none, some or all sectors are protected.
It is possible to globally (un)lock all sectors at once but in a way that was
not anticipated when refactoring the spi25 unlocking functions into
spi_disable_blockprotect_generic(). To globally unprotect not only the
protection bits (2 and 3) have 0 to be written to them but also bits 4 and 5
which normally would not be touched by spi_disable_blockprotect_generic().
Some of the chips also support a permanent lockdown with fuses which we
do not handle yet.

To fix this without copying the whole method I introduce another mask
parameter to spi_disable_blockprotect_generic() namely unprotect_mask.
See verbose comments inline for details.

Also, prettyprint the status register after trying to disable the block
protection fails.

Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Tested-by: Chi Zhang <zhangchi866@gmail.com>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://coreboot.org/flashrom/trunk@1679 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1