Read all 16 bits in readPhyByte, not just upper 8 bits

[JerthG]

readPhyByte would only read the upper 8 bits of the MIRD register. Now it should read all 16 bits.

[njh]

Thanks! Why was this useful for you?

[JerthG]

I was fiddling around trying to use your library to create my own with only the functionality i needed.
And i wanted to read the PHID1 and PHID2 registers to make sure my SPI connection was working.

[JerthG]

I see now that it broke isLinkUp, fixed that as well