mmc: dw_mmc: eliminate useless usage of ret

In few places usage of ret variable is not needed.
This patch simplifies those pieces of code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>