mfd: Fix stmpe section mismatch

This fixes:

WARNING: drivers/built-in.o(.text+0xf368f): Section mismatch in reference from
the function stmpe_probe() to the function .devinit.text:stmpe_chip_init()
The function stmpe_probe() references the function __devinit stmpe_chip_init().

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>