sfc: Make PHY flash mode a device attribute, not a module parameter

This allows updating PHY firmware for one interface without removing
all other interfaces handled by the driver.

Replace tx_disabled flags and 10Xpress status enumeration with flags in
enum efx_phy_mode.

Prevent an interface from being brought up while in PHY flash mode.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>