usb: gadget: add a new quirk to avoid skb_reserve in u_ether.c

Some platforms (e.g. USB-DMAC on R-Car SoCs) has memory alignment
restriction. If memory alignment is not match, the usb peripheral
driver decides not to use the DMA controller. Then, the performance
is not good.

In the case of u_ether.c, since it calls skb_reserve() in rx_submit(),
it is possible to cause memory alignment mismatch.

So, this patch adds a new quirk "quirk_avoids_skb_reserve" to avoid
skb_reserve() calling in u_ether.c to improve performance.

A peripheral driver will set this flag and network gadget drivers
(e.g. f_ncm.c) will reference the flag via gadget_avoids_skb_reserve().

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>