mmc: Extend wakelock if bus is dead

This patch sets extend_wakelock if the host's
bus is marked dead due to card removal.

This change was originally made in bc4dc52f
(mmc: mmcblk: Add support for deferred SD bus resume),
which was reverted due to the majority of the patch
no longer building or making sense. However one small
part of that patch ought to be saved.

Signed-off-by: John Stultz <john.stultz@linaro.org>