net: sfp: move sm_mutex into sfp_check_state()

Provide an unlocked version of sfp_sm_event() which can be used by
sfp_check_state() to avoid having to keep re-taking the lock if
several signals have changed state.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>