cxgbe(4): Tidy up t4_l2t.h

Align the comment for if_t with the rest and remove the declaration of
t4_l2t_set_switching, which was removed in 061bbaf7e7fe.

No functional change.

Sponsored by: Chelsio Communications

(cherry picked from commit b3bbc6cc79a3b76d342b2a8fd68e5266eb50ff11)