MMC: OMAP: Use tasklet instead of workqueue for cover switch notification

The cover waitqueue is occasionally scheduled twice from timer
and the interrupt and oops follows. It would have been possible
to fix this problem with spinlocks but using tasklet was a dropin
sloution with no need for locking.

This path also adds some cleanups.

Signed-off-by: Jarkko Lavinen <jarkko.lavinen@nokia.com>
Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>