net: sfp: re-attempt probing for phy

Some 1000BASE-T PHY modules take a while for the PHY to wake up.
Retry the probe a number of times before deciding that the module has
no PHY.

Tested with:
 Sourcephotonics SPGBTXCNFC - PHY takes less than 50ms to respond.
 Champion One 1000SFPT - PHY takes about 200ms to respond.
 Mikrotik S-RJ01 - no PHY

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>