rtw89: fix maybe uninitialized `qempty` variable

Call to dle_dfi_qempty might fail, leaving qempty.qempty untouched, which
is latter used to control the for loop. If that happens, it's not
initialized anywhere.

Initialize it so the loop doesn't iterate unless it's modified by the
call to dle_dfi_qempty.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220113094253.73370-1-ihuguet@redhat.com