drivers: ethernet: fixed calling mutex from ISR

The NXP ENET driver used a mutex inside an ISR.
Since mutexes are not allowed in ISR, an assertion had been triggered.
The mutex is not required since a semaphore ensures sequential writing
(in ISR) and subsequent reading (after eth_wait_for_ptp_ts()).
Fixed by removing calls to mutex.

Signed-off-by: Frank Duerr <frank.duerr@ipvs.uni-stuttgart.de>