stm32/modnwwiznet5k: Increase SPI bus speed to 42MHz.

The W5200 and W5500 can support up to 80MHz so 42MHz (the maximum the
pyboard can do in its standard configuration) should be safe.

Tested to give around 1050000 kbytes/sec TCP download speed on a W5500,
which is about 10% more than with the previous SPI speed of 21MHz.