USB: bcma: Make GPIO explicitly optional

What the code does is to not check the return value from
devm_gpiod_get() and then avoid using an erroneous GPIO descriptor
with IS_ERR_OR_NULL().

This will miss real errors from the GPIO core that should not be
ignored, such as probe deferral.

Instead request the GPIO as explicitly optional, which means that
if it doesn't exist, the descriptor returned will be NULL.

Then we can add error handling and also avoid just doing this on
the device tree path, and simplify the site where the optional
GPIO descriptor is used.

There were some problems with cleaning up this GPIO descriptor
use in the past, but this is the proper way to deal with it.

Cc: Rafał Miłecki <rafal@milecki.pl>
Cc: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/20221107090753.1404679-1-linus.walleij@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>