mmc: vt8500: Remove erroneous __exitp in wmt_mci_driver

With the __devinit/__devexit attributes having been removed, this
__exitp attribute causes an unused function warning and should be
removed as well.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Chris Ball <cjb@laptop.org>