iwlwifi: mvm: add a get lmac id function

As some functions need to check which lmac id to use, add
a new function to find the lmac id instead of open coding it
in every function.

Signed-off-by: Nathan Errera <nathan.errera@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20201008180656.ac3b43e97aa8.I308f8ed2238e32d5eef9c35ad7962f0e8b9c1902@changeid