drivers: slip: Skip buffer allocation for incomplete packet

If slip_input_byte fails to get buffer for the first byte then no point
of saving later bytes and send it to upper layers. Final packet will be
incomplete and upper layers will discard it. Consider incoming bytes
only after successful buffer allocation on first byte, otherwise silently
ignore it.

Change-Id: Ie16d0df0c608d1644d39f66900252a340051c012
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>