staging: mt7621-mmc: Remove unused card_workqueue from msdc_host

The card_workqueue field of msdc_host is already if 0'd out and there
are no references to it in the code (not even in unused code), so
remove it.

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