wil6210: remove BACK RX and TX workers

WMI synchronous handling has changed and WMI calls that provide
a buffer for the reply are completed in the WMI interrupt context.
This allows sending the RX and TX BACK commands from the WMI event
handler without the need for the worker thread.
This is a better approach as it can decrease the handshake time
in the connect flow and prevent race conditions in case of fast
disconnects. An example for such a race is handling of wil_back_rx_handle
during a disconnect event, as wil_back_rx_handle is not protected by
the wil mutex and a disconnect can be handled after sta->status is
verified as connected.

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>