net: lan966x: Don't read RX timestamp if not needed

Whenever a frame was received to the CPU, the HW is timestamping the
frame. In the IFH(Inter Frame Header) it is found the nanosecond part
of the timestamps the SW is required to read from HW the second part.
But reading the second part it seems to be a expensive operations, so
so change this such to read the second part only when rx filter is
enabled.
Doing this change gives the RX a performance boost of ~70mbit.

before:
[  5]   0.00-10.01  sec   546 MBytes   457 Mbits/sec    0 sender

now:
[  5]   0.00-10.01  sec   652 MBytes   530 Mbits/sec    0 sender

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>