mmc: cb710: Return err value in cb710_wait_while_busy()

Fixes:

drivers/mmc/host/cb710-mmc.c: In function ‘cb710_wait_while_busy’:
drivers/mmc/host/cb710-mmc.c:182:6: warning: variable ‘err’ set but not
used [-Wunused-but-set-variable]

Signed-off-by: Chris Ball <cjb@laptop.org>
Acked-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>