wil6210: support multiple desc and status rings handling

Support multiple status rings:
- Move Tx status ring initialization code to separate function
- Add status ring id argument to relevant functions instead of
  relaying on wil->tx_sring_idx
- Store allocated Rx status ring id in wil->rx_sring_idx and use it
  instead of hard coded WIL_DEFAULT_RX_STATUS_RING_ID
- Configure interrupt moderation for multiple rings
- Allow allocation of descriptor ring without ctx array
- Set irq_mode upon Tx ring allocation

Enhance wil_wmi_cfg_def_rx_offload to enable/disable checksum offload.

Change-Id: If7d346dec2c1ba35cf377235d4c520eca11be166
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>