uart-gb: improve minor device number error checking

When alloc_minor() finds an available minor device number it
does not constrain the highest number desired.  Instead, it
relies on its caller, tty_gb_probe() to see if the returned
number indicates all minor numbers have been exhausted.

There are a couple problems with this--or rather with this
code.

First, if an allocation is attempted *after* GB_NUM_MINORS
is returned, a new number greater than (but not equal to)
GB_NUM_MINORS will be allocated, and that won't produce
any error condition.

Second, alloc_minor() can return an error code (like -ENOMEM).  And
its caller is only checking for GB_NUM_MINORS.  If an error code
is returned, tty_gb_probe() simply uses it.

Change alloc_minor() so it requests minor device numbers in the
range 0..(GB_NUM_MINORS-1), and use an error return to detect
when the minor device numbers have been exhausted.

If alloc_minor() returns -ENOSPC (from idr_alloc()), translate that
to -ENODEV.  The only other error we might see is -ENOMEM, and if
we get that, return it.

Finally, zero gb_tty->minor when it's released.  (If this is
actually important a reserved value like GB_NUM_MINORS should
be used instead to signify a gb_tty with no minor assigned.)

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
uart-gb: a few minor bug fixes

Here are a few small bug fixes in uart-gb.c:
    - In wait_serial_change():
        - Return -EINVAL if *none* of the relevant flags are set in
          the "arg" parameter.
        - Balance the spin_lock_irq() with an unlock call (not
          another lock).
        - Rearrange a nested if structure (not a bug fix).
    - In tty_gb_probe():
        - Reset the greybus_device driver data in case of error.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>