rt2800usb: check Autorun mode on FW load only once

Seems H/W report correctly firmware Autorun value only at initialization
stage. When we close interface and open it again, Autorun value is 0 and
we try to load firmware what kills the device. To fix clear
REQUIRE_FIRMWARE firmware flag, to do not load firmware again, once we
discover Autorun mode.

Reported-and-tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>