wireless/p54: prepare for FIRMWARE_NAME_MAX removal

We're going to remove the FIRMWARE_NAME_MAX definition in order to avoid any
firmware name length restriction.
This patch gets rid of the statically allocated p54usb firmware string, and
replaces them with const char pointers.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>