gpio: dwapb: fix pointer to integer cast

The statements BUG_ON(ctx == 0) was implicitly casting a pointer
to an integer for comparison. Do this with a bool test instead
to get away from sparse warnings.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

diff --cc drivers/gpio/gpio-dwapb.c
Simple merge