net: sfp: add sfp_modify_u8() helper

Add a helper to modify bits in a single byte in memory space, and use
it when updating the soft tx-disable flag in the module.

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>