net: sparx5: Fix invalid timestamps

Bit 270-271 are occasionally unexpectedly set by the hardware. This issue
was observed with 10G SFPs causing huge time errors (> 30ms) in PTP. Only
30 bits are needed for the nanosecond part of the timestamp, clear 2 most
significant bits before extracting timestamp from the internal frame
header.

Fixes: 70dfe25c ("net: sparx5: Update extraction/injection for timestamping")
Signed-off-by: Aakash Menon <aakash.menon@protempis.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>