libertas: support boot commands to write persistent firmware and bootloader

Add locking and non-locking versions of if_usb_prog_firmware to support
programming firmware after and before driver bring-up respectively.  Add more
suitable error codes for firmware programming process.  Add capability checks
for persistent features before attempting to use them.

Based on patches from Brajesh Dave and Priyank Singh.

Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>