serial: remove calls to serial_assign()

serial: remove calls to serial_assign()

Remove calls to serial_assign() that are failing now that it returns a
proper error code.  This calls were not actually doing anything
because they passed the name of a stdio_dev when a serial_device name
is exptectd.

Signed-off-by: Allen Martin <[email protected]>
Acked-by: Joe Hershberger <[email protected]>
Acked-by: Marek Vasut <[email protected]>
Acked-by: Simon Glass <[email protected]>
Tested-by: Simon Glass <[email protected]>
Tested-by: Stephen Warren <[email protected]>