Revert "serial_core: Update buffer overrun statistics."

This reverts commit 642180871bc91afebb6ccf40d1615a7dd33699a3.

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>