net: sfp: only use soft polling if we have A2h access

The soft state bits are stored in the A2h memory space, and require
SFF-8472 compliance. This is what our have_a2 flag tells us, so use
this to indicate whether we should attempt to use the soft signals.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>