iwlwifi: refactor txq_alloc for supporting more command type

Support more txq_alloc command types by moving the command declaration
to the gen specific area.  While at it, move some of the code segments
to a common place for re-use.

Signed-off-by: Golan Ben Ami <golan.ben.ami@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>