Wifi: Add ether type to sending offloaded packet

In current implementation, the ether type used for offloaded
packets is always hardcoded to IPv4 in lower layers.
This commit adds support to specify ether type of offloaded
packets from user space.

Bug: 122487582
Test: Manual
Change-Id: I1ca7391bd70ecebaf0591e0eafe08b79383538c5