mmc: omap_hsmmc: Use regulator_set_enable_if_allowed for enabling regulator

Use regulator_set_enable_if_allowed() api instead of regulator_set_enable()
while enabling io regulators. This way the driver doesn't see an error
when disabling an always-on regulator and when enabling is not supported.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>