mwl8k: use pci_unmap_addr{,set}() to keep track of unmap addresses on rx

Instead of reading back the unmap address from the receive
descriptor when doing receive processing, use DECLARE_PCI_UNMAP_ADDR
and pci_unmap_addr{,set}() to keep track of these addresses.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>