staging: mt7621-pci: rewrite RC FTS configuration

The RC FTS configuration is done using hardcoded registers
and bitshift operations. Make it a bit clean defining some
prepocessor definitions and simple macros.

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