mimxrt/eth: Avoid timeout in LAN constructor.

[Romaric-RILLET]

Summary

When there is no ethernet cable connected to board, avoid fail/timeout in LAN constructor.
It permit to use the status method of the LAN object to check if the physical Ethernet link is connected.

Testing

Tested with Teensy 4.1 board

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32