b43: Enforce DMA descriptor memory constraints

Enforce all device constraints on the descriptor memory region.

There are several constraints on the descriptor memory, as documented
in the specification. The current code does not enforce them and/or
incorrectly enforces them.

Those constraints are:
- The address limitations on 30/32bit engines, that also apply to
  the skbs.
- The 4k alignment requirement on 30/32bit engines.
- The 8k alignment requirement on 64bit engines.

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