staging: mt7621-mmc: Remove MSDC_MODE_* defines

The MSDC_MODE_* defines are all never used and do not contain any
information about the device, so remove them.

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