mmc: dw_mmc: allow probe to succeed even if one slot is initialized

Instead of aborting the probe when a slot initialization fails, allow
initialization of as many slots as possible. If there is at least one
instance of a slot that is successfully initialized, allow the driver
probe to succeed.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>