mmc: core: add mmc flush detect work.

Block device should be available before mounting of
root device. In some cases block device is not available
at the time of root device mounting because mmc detect
work is not completed by that time which is causing mount
failure. To fix this, flush mmc detect work after device
probe completion.

Change-Id: I785a1e01a691652cfdb5a82acee2475e23fe3766
Signed-off-by: Ankit Jain <jankit@codeaurora.org>