mmc: mxs: use standard flag for broken card detection

Use the standard MMC_CAP_NEEDS_POLL flag rather than a custom broken_cd
flag. The original code used to just return true in the card detection
function for broken card detection. The MMC_CAP_NEEDS_POLL works different,
but was introduced for the same purpose, so assume the code works correct
now.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>