serial: 8250: Use defined IER bits

Instead of literal 0x0f, add a define for enabling all IER bits the
8250 driver is interested in.

Don't make the define for combined flags part of UAPI.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221125130509.8482-2-ilpo.jarvinen@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>