staging: et131x: Remove fbr_lookup.real_physaddr

fbr_lookup.real_physaddr is only being used as a pseudonym for
fbr_lookup.ring_physaddr, so remove it and rename all instances to
ring_physaddr.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>