net: cdc_ncm: simplify and optimize frame padding

We can avoid the costly division for the common case where
we pad the frame to tx_max size as long as we ensure that
tx_max is either the device specified dwNtbOutMaxSize or not
a multiplum of wMaxPacketSize.

Using the preconverted 'maxpacket' field avoids converting
wMaxPacketSize to CPU endianness for every transmitted frame

And since we only will hit the one byte padding rule for short
frames, we can drop testing the skb for tailroom.

The change means that tx_max now represents the real maximum
skb size, enabling us to allocate the correct size instead of
always making room for one extra byte.

Cc: Alexey Orishko <alexey.orishko@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>