serial: meson: remove use of flag UPF_IOREMAP

Flag UPF_IOREMAP is used by the 8250 subsystem only, it's not used
by the serial core. Therefore I don't see any benefit in using it
here.

In addition fix the order of calls in meson_uart_release_port.
Unmapping needs to be done first, reversing call order in
meson_uart_request_port.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>