mgmt/mcumgr/lib: Return error on bad SMP header

The changes smp_process_request_packet to return MGMT_ERR_ECORRUPT
in case when failed to read header.
This helps to recognize that there will not be any response sent
from mcumgr to the buffer given to smp_process_request_packet as
it has not been recognized as valid request.
It also removes redundant check on mgmt_streamer_init_reader which
always returns 0 (no failure) in Zephyr.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>