mmc: dw_mmc: Extract waiting for data busy into a separate routine

mmc: dw_mmc: Extract waiting for data busy into a separate routine

Waiting for data busy is a logically separate operation and should be
implemented as a separate routine. Follow Linux kernel example and
extract it from dwmci_send_cmd(). This way it doesn't clutter
dwmci_send_cmd() function, and can be reused later in other cases.

No functional change.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>