cxl: cxl_afu_reset() -> __cxl_afu_reset()

Rename cxl_afu_reset() to __cxl_afu_reset() to we can reuse this function name
in the API.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>