libcxgb,iw_cxgb4,cxgbit: add cxgb_best_mtu()

Add cxgb_best_mtu() in libcxgb_cm.h to remove
it's duplicate definitions from cxgb4/cm.c and
cxgbit/cxgbit_cm.c

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>