mwl8k: use the dma state API instead of the pci equivalents

The DMA API is preferred.

No functional change.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>