tty: serial: extract uart_change_port() from uart_set_info()

tty: serial: extract uart_change_port() from uart_set_info()

This "change_port" part of uart_set_info() is for no good reason
inlined there. It makes the function rather hard to read. Therefore,
extract it to a separate function.

This allows for flattening the ifs (with short path "return"s) and
avoiding two levels of indentation. Both making the code really flat and
comprehesible.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20241211074933.92973-4-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>