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: NRussell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>