net: sfp: add helper to modify signal states

There are a couple of locations in the code where we modify
sfp->state, and then call sfp_set_state(, sfp->state) to update
the outputs/soft state to control the module. Provide a helper
which takes a mask and new state so that this is encapsulated in
one location.

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>