staging: mt7621-pci: reorder preprocessor definitions

Reorder a bit register definitions and some values into
logical groups adding some comments for each group. Also
unify all of register to use 16 bit in definitions instead
of having some with 16 bits and some with 8 bits.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>