MFD: SM501 debug typo fix

Remove errnoeous x character from dev_dbg() call that stops the driver
compiling under debug.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>