mmc: tmio: refactor .get_ro hook

This IP provides the write protect signal level in the status
register, but it is also possible to use GPIO for WP.  They are
exclusive, so it is not efficient to call mmc_gpio_get_ro() every
time from tmio_mmc_get_ro() if we know gpio_ro is not used.

Check the capability of gpio_ro just once in the probe function,
then set mmc_gpio_get_ro to .get_ro if it is the case.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>