libata-eh.c: local functions should not be exposed globally

The function ata_ering_clear_cb is only referenced in this file and
should be marked static to prevent it from being exposed globally.

This quiets the sparse warning:

warning: symbol 'ata_ering_clear_cb' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>