staging: mt7621-mmc: Remove register debugging structures

Current code has structures for all the registers of the device, but
these are never used and there are also masks for all of them, so
these structures do not contain any useful information.

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