The Ethernet module of the MCU requires that the RDAR register is set
only after a while after the RDSR register is set; i.e. the RDAR
register should not be set immediately after RDSR during initialization
of the Ethernet module.
  1. If we write to the ENET_RDAR register immediately after initializing
the ENET_RDSR register, the incoming packets can _never_ be received.
  2. If we make the MCU wait for 10us before writing to ENET_RDAR, the
incoming packets are _always_ received correctly.
  3. If we perform a few instructions (about 10 of them) between setting
RDSR and RDAR, then the problem manifests randomly from time to time.

In order to fix the bug, we add a 10us delay just before writing
to the RDAR register.

Things just work in Freescale MQX and in Linux, because there is a lot
of other initialization code between the RDAR and RDSR register are set,
so this other initialization code serves as a delay.