mmc: dw_mmc: Check return value of regulator_enable

regulator_enable() is declared with __must_check attribute.
Hence check the return value to ensure that the regulator is enabled.

Fixes the following warning:

drivers/mmc/host/dw_mmc.c:2461:19: warning:
ignoring return value of ‘regulator_enable’, declared with attribute
warn_unused_result [-Wunused-result]
drivers/mmc/host/dw_mmc.c: In function ‘dw_mci_init_slot’:
drivers/mmc/host/dw_mmc.c:1994:19: warning:
ignoring return value of ‘regulator_enable’, declared with attribute
warn_unused_result [-Wunused-result]

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>