serial: core: Expand port mutex section in uart_poll_init()

Prepare uart_poll_init() to safely dereference uart port; expand the
port mutex section to guarantee uart port remains valid until
uart_poll_init() completes.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>