term/serial: Avoid double lookup of serial ports

term/serial: Avoid double lookup of serial ports

The various functions to add a port used to return port->name, and
the callers would immediately iterate all registered ports to "find"
the one just created by comparing that return value with ... port->name.

This is a waste of cycles and code. Instead, have those functions
return "port" directly.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>