mmc: block: switch card to User Data Area when removing the block driver

The MMC block driver and other drivers (e.g. mmc-test) will expect
the card to be switched to the User Data Area eMMC partition when
they start.  Hence the MMC block driver should ensure it is that
way when it is removed.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andrei Warkentin <andreiw@motorola.com>
Signed-off-by: Chris Ball <cjb@laptop.org>