mhi_bus: devices: netdev: remove submitted skb list

MHI bus driver will always do a callback for each buffer that's
queued to hardware. So, no need to keep track of the submitted
buffers using submitted list, this patch removes the submitted
skb list, and related locks.

CRs-Fixed: 2221018
Change-Id: Icc9b61c788e96c57409afa9578025251a146c2cc
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>