In uart_cnprobe(), fill in the cn_name field of the consdev structure.

In uart_cnprobe(), fill in the cn_name field of the consdev structure.
The core console code checks this field when a console is added and
emits a warning if it's empty. In practice the warning is harmless for
uart(4), because the cn_name is filled in as soon as the device name is
known; which is when the device is enumerated.
To avoid the warning, to avoid possible complications caused by emitting
the warning without there (possibly) being a console selected yet and to
avoid complications when the UART isn't found during bus enumeration, we
just preset the cn_name field here to the name of the driver.