drivers: ethernet: phy: microchip_t1s: always reschedule phy_monitor_work

Previously, phy_monitor_work_handler() would return early without
rescheduling the delayed work if the callback (cb) was not set,
causing the periodic monitoring to stop. This change ensures that
k_work_reschedule() is always called, even when cb is NULL, so
monitoring of the PHY state continues.

This prevents the monitor from being inadvertently stopped and
ensures consistent behavior regardless of callback registration.

This issue was observed during testing with the evb-lan8670-rmii
(an external LAN8670 PHY) connected to the SAME54 Curiosity Ultra
platform.

Signed-off-by: Parthiban Veerasooran <parthiban.veerasooran@microchip.com>