tty: serial: get rid of exit label from uart_set_info()

tty: serial: get rid of exit label from uart_set_info()

The label is unneeded since 7ba2e769825f (tty: Split the serial_core
helpers for setserial into two). Until then, there was a lock held in
uart_set_info().

Now it is not, so we can remove the label. This involves reordering the
code, so that it is clear what values are returned, where and why. Until
now, it was really hard to follow.

The "change_port" part of the function is extracted into a separate
function in the next patch. This patch makes the transition there easier
too.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20241211074933.92973-3-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>