staging: mt7621-mmc: Cleanup source of base address for io

Currently the base address for all io operations is hidden behind a
macro, REG_ADD. This macro uses the symbol "base" as the base address
and all functions set base = host->base. This is hard to read, so the
whole wrapping is removed and host->base is directly inserted in the
io access.

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