stm32_ifdown() holds critical section when calling stm32_ethreset().
That function used to call up_mdelay(10) while waiting for the ethernet
peripheral reset to complete. This resulted in excessively long
critical section time with interrupts disabled.
The actual expected delay is a few clock ticks of the 50 MHz clock domain.
This commit changes polling interval to 1us and maximum to 10us.
d68c8ec560