net: stmmac: use correct PPS capture input index

The stmmac supports up to 4 auxiliary snapshots that can be enabled by
setting the appropriate bits in the PTP_ACR bitfield.

Previously as of commit f4da5652

 ("net: stmmac: Add support for
external trigger timestamping") instead of setting the bits, a fixed
value was written to this bitfield instead of passing the appropriate
bitmask.

Now the correct bit is set according to the ptp_clock_request.extts_index
passed as a parameter to stmmac_enable().

Signed-off-by: Johannes Zink <j.zink@pengutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>