hyperv: Fix the max_xfer_size in RNDIS initialization

According to RNDIS specs, Windows sets this size to
0x4000. I use the same value here.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>