ar9170: add AVM FRITZ devices

This adds:

USB 0x057C:0x8401       AVM FRITZ!WLAN USB Stick N
USB 0x057C:0x8402       AVM FRITZ!WLAN USB Stick N 2.4

These devices require the 1-stage firmware, if not present we
don't continue.

Cc: Peter Grabienski <Peter.Grabienski@Atheros.com>
Cc: Stephen Chen <Stephen.Chen@Atheros.com>
Cc: Michael Fortin <Michael.Fortin@Atheros.com>
Cc: Johnny Cheng <Johnny.Cheng@Atheros.com>
Cc: Yuan-Gu Wei <Yuan-Gu.Wei@atheros.com>
Cc: Joerg Albert <jal2@gmx.de>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>