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 <amartin@nvidia.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>