wil6210: make sure Rx ring sizes are correlated

When enlarging rx_ring_order module param, wil6210 fails to load
because there are not enough Rx buffers.
Fix this by enlarging number of Rx buffers at startup, if needed based
on rx_ring_order.

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>