serial, x86: use UPF_* constants for flags

This patch fixes the following sparse warnings:

drivers/tty/serial/8250/8250_core.c:3231:32: warning: incorrect type in assignment (different base types)
drivers/tty/serial/8250/8250_core.c:3231:32:    expected restricted upf_t [usertype] flags
drivers/tty/serial/8250/8250_core.c:3231:32:    got unsigned int const [unsigned] flags

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>