rsu: various initialization fixes.

• Do not ignore initialization errors; call ieee80211_stop()

when initialization failed.

• Use usb_pause_mtx() instead of DELAY() while waiting for firmware

loading; this fixes system freeze during firmware startup.

• Do not execute rsu_stop() when device is powered off; fixes

'unknown board type (rfconfig=0xff)' error when the device is
reattached.

Tested with Asus USB-N10.