b43: Rename the DMA ring pointers

Rename the DMA ring pointers to have more descriptive and standard
names. Also remove the 6th unused TX ring. We can add it back later,
if we need it. The unused TX-status rx-ring is also removed, as that's
only used by legacy devices not supported by this driver anyway.

This is no functional change, except less memory allocation for
the removed rings.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>