iwlwifi: mvm: Enable Rx Checksum hw

TCP software implementation on the host requires extensive computing
power.  Offloading even some of the TCP/IP stack to the NIC might save
a significant overhead. In order to enable this feature on our hw,
we need to configure it first. Once done, we mark this capability,
to be advertised later to the OS via ieee80211_register_hw.
The driver Rx indications for TCP Checksum is integrated within the
standard Rx status. The driver responds to those indications as follows:
If the frame was tested by hw and checksum ok report CHECKSUM_UNNECESSARY.
Otherwise, report CHECKSUM_NONE.

Signed-off-by: Avri Altman <avri.altman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>