net: fec: consolidate hwtstamp implementation

Both transmit and receive use the same infrastructure for calculating
the packet timestamp.  Rather than duplicating the code, provide a
function to do this common work.  Model this function in the Intel
e1000e version which avoids calling ns_to_ktime() within the spinlock;
the spinlock is critical for timecounter_cyc2time() but not
ns_to_ktime().

Acked-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>