rt2x00: Remove redundant flags/dev_flags initializations

the rxdesc structure is properly memsetted before passed to
the driver. This means we don't have to reinitialize the flags
and dev_flags fields in the drivers again.
This will prevent problems when the rxdone handler is adding
flags in a earlier status and will make the code look nicer
when we are adding more read attributes in the rxdone handler
in the driver.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>