Make SPI access atomic in Ethernet library

When using the Ethernet library combined with another library that does
SPI access inside an interrupt handler, the Arduino could crash if that
interrupted occured while the Ethernet library was doing an SPI
transfer.

This commit makes all Ethernet SPI transfers happen atomically (i.e.,
with the global interrupt flag cleared), preventing such conflicts by
delaying all interrupts until after the SPI transfer.

Closes: #384