net: sfp: allow sfp to probe slow to initialise GPON modules

Some GPON modules (e.g. Huawei MA5671A) take a significant amount of
time to start responding on the I2C bus, contary to the SFF
specifications.

Work around this by implementing a two-level timeout strategy, where
we initially quickly retry for the module, and then use a slower retry
after we exceed a maximum number of quick attempts.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>