mmc: core: Rework wp-gpio handling

Use MMC_CAP2_RO_ACTIVE_HIGH flag as indicator if GPIO line is to be
inverted compared to DT/platform-specified polarity. The flag is not used
after init in GPIO mode anyway. No functional changes intended.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lore.kernel.org/r/a60f563f11bbff821da2fa2949ca82922b144860.1576031637.git.mirq-linux@rere.qmqm.pl
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>