mmc: print an error on FLUSH timeout

Up until now the FLUSH timeout error was not printed and there
was no way to know if the FLUSH completed successfully or not.
Printing the FLUSH failure will help to identify if FS corruption
happened due to this timeout.

Change-Id: I352423c84e2ca698284b90af5837ba96d03919bf
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>