staging: mt7621-mmc: Remove code for not existent config

The MT7621|7628_FPGA config flags are non existent in the tree, so
code guarded by this flags can be removed.

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