net: stmmac: use __napi_schedule() for PREEMPT_RT

Use of __napi_schedule_irqoff() is not safe with PREEMPT_RT in which
hard interrupts are not disabled while running the threaded interrupt.

Using __napi_schedule() works for both PREEMPT_RT and mainline Linux,
just at the cost of an additional check if interrupts are disabled for
mainline (since they are already disabled).

Similar to the fix done for enetc commit 215602a8

 ("enetc: use
napi_schedule to be compatible with PREEMPT_RT")

Signed-off-by: Seb Laveze <sebastien.laveze@nxp.com>
Link: https://lore.kernel.org/r/20210112140121.1487619-1-sebastien.laveze@oss.nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>