mfd: stmfx: Fix macro definition spelling

Clang warns:

In file included from drivers/mfd/stmfx.c:13:
include/linux/mfd/stmfx.h:7:9: warning: 'MFD_STMFX_H' is used as a
header guard here, followed by #define of a different macro
[-Wheader-guard]

Fixes: 06252ade

 ("mfd: Add ST Multi-Function eXpander (STMFX) core driver")
Link: https://github.com/ClangBuiltLinux/linux/issues/475
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>