iwlwifi: mvm: refactor setting PPE thresholds in STA_HE_CTXT_CMD

We are setting the PPE Thresholds in STA_HE_CTXT_CMD according
to HE PHY Capabilities IE. As EHT is introduced, we will have to
set this thresholds according to EHT PHY Capabilities IE if we're
in an EHT connection. Some parts of the code can be used for both
HE and EHT. Put this parts in functions which will be used in the
patch which adds support for EHT PPE.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20220205112029.48a508dfffef.If392e44d88f96ebed7fadf827e327194d4bd97b1@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>