iwlwifi: mvm: cleanup incorrect and redundant define

Currently we have up to 3 phy contexts - defined by NUM_PHY_CTX.
However - some code paths validate the ID by using MAX_PHYS define
which is set to 4.
While there is no harm it is incorrect - since the maximum is 3.
Remove the define and use the correct one.
Cleanup the code a bit while at it.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>