rt2800: correct AUTO_RSP_CFG

Initialize AUTO_RSP_CFG register to similar value as vendor driver does.

Do not set BAC_ACK_POLICY based on short preamble setting, those are
unrelated.

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