[PATCH] rt2x00: Fix residual check in PLCP calculations.

Because rt2x00 implements the residual calculation different from what the
legacy code does (i.e. scaled values), we need to adjust the residual check.
Again, we are only mimicking the behaviour of the ralink driver without
actually knowing what we do :-(

Signed-off-by: Mattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>