platform: cznic: use ffs() instead of __bf_shf()

__bf_shf() on a 64-bit variable causes a link failure during
compile-testing:

drivers-platform-cznic-turris-omnia-mcu-gpio.c:(.text):undefined-reference-to-__ffsdi2

Open-code this using ffs()-1, which has the same result but avoids
the library call.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>