Revert "serial_core: Update buffer overrun statistics."

This reverts commit 64218087

.

Buffer overruns are for hardware reported overruns, not software ones,
which will only happen if we run out of memory and you will get lots of
-ENOMEM errors at the same time.

Thanks to Alan Cox for catching this.

Reported-by: Alan Cox <alan@linux.intel.com>
Cc: Corbin Atkinson <corbinat@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>