mmc: omap_hsmmc: use slot-gpio functions to manage read-only pin directly

The indirection via omap_hsmmc_get_ro and omap_hsmmc_get_wp is
redundant. Also dropped setting gpio_wp to EINVAL since platform date
is read-only
Untested: no device with ro pin was available, but change is fairly
simple

Signed-off-by: Andreas Fenkart <afenkart@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>