staging: mt7621-mmc: Do not BUG() if mrq set in msdc_ops_request

Currently the code BUG()'s, if host->mrq is set at the beginning of
msdc_ops_request. This shoould normally not happen, but it is not that
critical, because the critical sections are protected by a spin lock
and in the worst case, some commands to the card are lost, so it is
sufficient to just WARN_ON().

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>