net: enetc: Replace ifdef with IS_ENABLED

net: enetc: Replace ifdef with IS_ENABLED

The enetc driver uses ifdefs when checking whether
CONFIG_FSL_ENETC_PTP_CLOCK is enabled in a number of places. This works
if the driver is built-in but fails if the driver is available as a
kernel module. Replace the instances of ifdef with use of the IS_ENABLED
macro, that will evaluate as true when this feature is built as a kernel
module and follows the kernel's coding style.

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240912173742.484549-1-martyn.welch@collabora.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>