USB: gadget: u_ether: unalign IP header to make DMA aligned

NET_IP_ALIGN value 2 helps to align the IP packet buffer
and this may help CPU handling of IP packets. But, this
causes the RNDIS RX buffer pointer to become unaligned.
Some USB controller DMA engines may expect aligned buffers
for efficient data transfers, otherwise takes extra cycles
and may cause performance penalty. Chip Idea USB controller
is one in this category. With this change, observed RNDIS
performance improvement by around 10Mbps in TCP Bi-directional
tests.

Change-Id: I3a3874899d1db04c0065c7b5f065e39f3359f3d9
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>