msm: sdio: read the HW mailbox in sdio_close flush procedure

In the sdio_close flush procedure we need to make sure read_mailbox
is done to verify that the rx bytes avail were updated, before we can
complete the flush.
In case the apps is loaded, we cannot assume that read_mailbox was done
during the wait time and we need to explicitly call read_mailbox.

Signed-off-by: Maya Erez <merez@codeaurora.org>