net: dm: fec: Fix time unit error in phy-reset-duration

The DT binding says that phy-reset-duration is in ms, but the driver
currently uses udelay().

Switch to mdelay() to fix this.

Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>