gpio-mxc: stop including <asm-generic/bug>

<asm-generic/bug> contains the default implementation of BUG() and friends,
which architectures may decide to use.  The proper way to get them is
<linux/bug.h>, so use that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>