b43legacy: fix DMA slot resource leakage

This fixes four resource leakages.
In any error path we must deallocate the DMA frame slots we
previously allocated by request_slot().
This is done by storing the ring pointers before doing any ring
allocation and restoring the old pointers in case of an error.

This patch by Michael Buesch has been ported to b43legacy.

Cc: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>