mwl8k: change pci id table driver data to a structure pointer

To prepare for adding support for more device types, introduce a
new structure, mwl8k_device_info, where per-device information can
be stored, and change the pci id table driver data from an integer
indicating only the part number to a pointer to a mwl8k_device_info
structure.

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