mfd: stmpe: mask off unused blocks properly

The STMPE driver would just read/modify/write the system control
register on the STMPE1601, meaning it would not properly mask off
the PWM block, which remained active if it was on at boot time.
This makes sure the blocks are always masked off if they were
active on boot, saving some power. Also rename the inconsistenty
named STMPE1601 define for the PWM block activation.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>