net: ipa: rename endpoint->trans_tre_max

The trans_tre_max field of the IPA endpoint structure is only used
to limit the number of fragments allowed for an SKB being prepared
for transmission.  Recognizing that, rename the field skb_frag_max,
and reduce its value by 1.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>