net: fec: avoid lock evasion when reading pps_enable

net: fec: avoid lock evasion when reading pps_enable

The assignment of pps_enable is protected by tmreg_lock, but the read
operation of pps_enable is not. So the Coverity tool reports a lock
evasion warning which may cause data race to occur when running in a
multithread environment. Although this issue is almost impossible to
occur, we'd better fix it, at least it seems more logically reasonable,
and it also prevents Coverity from continuing to issue warnings.

Fixes: 278d24047891 ("net: fec: ptp: Enable PPS output based on ptp clock")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Link: https://lore.kernel.org/r/20240521023800.17102-1-wei.fang@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>