staging: mt7621-mmc: Remove unused macros sdr_{read,write}16

The sdr_{read,write}16 macros are never used and does not provide any
information about the device, so they are removed.

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