Recent change to this core breaks arduino recommended SerialFlash Library

[sambartle]

The following commit causes problems with the SerialFlash library recommended by Arduino for MKR MEM Shield: 738a0e9

I assume either changes may need to be made to the library to accommodate this fix to the core, or a tweak to the change here to accomodate the library.. but it is beyond my current level of understanding how to resolve unfortunately.

See: PaulStoffregen/SerialFlash#79 for full details.

[facchinm]

Hi @sambartle ,
the issue should be fixed by #622 , if you could test it and report back it would speedup the merge.