p54usb: Load firmware asynchronously

Drivers that load firmware from their probe routine have problems with
the latest versions of udev as they get timeouts while waiting for user
space to start. The problem is fixed by using request_firmware_nowait()
and delaying the start of mac80211 until the firmware is loaded.

To prevent the possibility of the driver being unloaded while the
firmware loading callback is still active, a completion queue entry
is used.

Also, to simplify the firmware loading procedure, this patch removes
the old, unofficial and confusing fallback firmware names. However,
they are still supported! So any user - who is still using them -
is hereby advised to link/rename their old firmware filenames:
	isl3890usb to isl3886usb
	isl3887usb_bare to isl3887usb

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>