u_ether: Handle memory allocation failure case on tx path

With multipacket feature enabled, if the memory allocation for
tx request buffer fails, accessing the req->buf leads to null
pointer dereference.

Hence, handle memory alloction failure case and return -ENOMEM.

Change-Id: Ic5faac8ea7eb056755b03007c48ff085f10afa20
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>