drivers: ethernet: eth_sam_gmac: Fix ptp adjust

NSEC_PER_SEC is an unsigned literal which will promote variable increment
to unsigned for the comparison operation, thus returning -EINVAL for
negative increment values.

For positive increment, -NSEC_PER_SEC becomes a large unsigned value
which will also return -EINVAL.

Fix by casting NSEC_PER_SEC to an int.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>