mmc: tmio: fix compiler warning

This patch fixes a compiler warning:

drivers/mmc/host/tmio_mmc_pio.c: In function 'tmio_mmc_power_on':
drivers/mmc/host/tmio_mmc_pio.c:798:19: warning: ignoring return value of
'regulator_enable', declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>