net: sfp: re-implement soft state polling setup

Re-implement the decision making for soft state polling. Instead of
generating the soft state mask in sfp_soft_start_poll() by looking at
which GPIOs are available, record their availability in
sfp_sm_mod_probe() in sfp->state_hw_mask.

This will then allow us to clear bits in sfp->state_hw_mask in module
specific quirks when the hardware signals should not be used, thereby
allowing us to switch to using the software state polling.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>