mmc: core: Fix PowerOff Notify suspend/resume

Modified the mmc_poweroff to resume before sending the poweroff
notification command. In sleep mode only AWAKE and RESET commands are
allowed, so before sending the poweroff notification command resume from
sleep mode and then send the notification command.

PowerOff Notify is tested on a Synopsis Designware Host Controller
(eMMC 4.5). The suspend to RAM and resume works fine.

Signed-off-by: Girish K S <girish.shivananjappa@linaro.org>
Tested-by: Girish K S <girish.shivananjappa@linaro.org>
Reviewed-by: Saugata Das <saugata.das@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>