usbnet: do not pretend to support SG/TSO

usbnet doesn't support yet SG, so drivers should not advertise SG or TSO
capabilities, as they allow TCP stack to build large TSO packets that
need to be linearized and might use order-5 pages.

This adds an extra copy overhead and possible allocation failures.

Current code ignore skb_linearize() return code so crashes are even
possible.

Best is to not pretend SG/TSO is supported, and add this again when/if
usbnet really supports SG for devices who could get a performance gain.

Based on a prior patch from Freddy Xin <freddy@asix.com.tw>

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>