net: sfp: get rid of DM7052 hack when enabling high power

Since we no longer mis-detect high-power mode with the DM7052 module,
we no longer need the hack in sfp_module_enable_high_power(), and can
now switch this to use sfp_modify_u8().

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>