net: sfp: split power mode switching from probe

Switch the power mode switching from the probe, so that we don't
repeatedly re-probe the SFP device if there is a problem accessing
the registers at I2C address 0x51.

In splitting this out, we can also fix a bug where we leave the module
in high-power mode when the upstream device is detached but the module
is still inserted.

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