net: cdc_ncm: no not set tx_max higher than the device supports

There are MBIM devices out there reporting

  dwNtbInMaxSize=2048 dwNtbOutMaxSize=2048

and since the spec require a datagram max size of at least
2048, this means that a full sized datagram will never fit.

Still, sending larger NTBs than the device supports is not
going to help.  We do not have any other options than either
 a) refusing to bindi, or
 b) respect the insanely low value.

Alternative b will at least make these devices work, so go
for it.

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