net: sfp: check firmware provided max power

Check that the firmware provided maximum power is at least 1W, which
is the minimum power level for any SFP module.

Now that we enforce the minimum of 1W, we can exit early from
sfp_module_parse_power() if the module power is 1W or less.

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>