net: sfp: add debugfs support

Add debugfs support to SFP so that the internal state of the SFP state
machines and hardware signal state can be viewed from userspace, rather
than having to compile a debug kernel to view state transitions in the
kernel log.  The 'state' output looks like:

Module state: empty
Module probe attempts: 0 0
Device state: up
Main state: down
Fault recovery remaining retries: 5
PHY probe remaining retries: 12
moddef0: 0
rx_los: 1
tx_fault: 1
tx_disable: 1

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1kyYRe-0004kN-3F@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>