staging: mt7621-pci: use BIT macro in preprocessor definitions

Some preprocessor definitions are using a custom implementation of
BIT macro. Just use linux kernel BIT macro instead.

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