mmc: sdhci: Don't ignore regulator_enable() return value

Fixes:
/git/arm-soc/drivers/mmc/host/sdhci.c: In function 'sdhci_add_host':
/git/arm-soc/drivers/mmc/host/sdhci.c:2910:19: warning: ignoring
return value of 'regulator_enable', declared with attribute
warn_unused_result [-Wunused-result]

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chris Ball <cjb@laptop.org>