rt2800: fix assigning same WCID for different stations

On some hardware reading WCID entries table results getting 0xff
numbers, no matter of value written there before. This cause assigning
the same WCID for different stations and makes not possible to connect
to more than one station.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>