mmc: sdhci: introduce get_ro private write-protect hook

Some controllers handle their write-protection differently. Introduce a
callback to be able to handle it, ensuring the same locking takes place
for it. Rename the status variable to make it more obvious why the read
from the registers needs to be inverted.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Chris Ball <cjb@laptop.org>