usb: gadget: u_ether: Add IOCTLs to allow MODE and HEADROOM changes

u_ether by default uses ETHERNET mode for packets. But some transports
e.g. RMNET could be using u_ether for IP packets. Hence, allow
QMI control bridge to change mode to IP from ETH. In IP mode, u_ether
wouldn't expose any mac address related operations and would set
protocol correctly as IPv4 or IPv6 instead of ETHERNET.
Headroom ioctl is required by clients to ask u_ether reserve headroom
for RX packets destined to clients which need to append some header
of their own without performing skb copy.

Change-Id: I868f2439524fb072786cfe765e92845a8154dfad
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>