Access to Serial flash memory on MKRWAN1310 fails under core v. 1.8.11

[terryzar]

Using the SerialFlash library, accessing the on board serial flash memory works fine under Arduino SAMD Core library v1.8.10. After upgrade to 1.8.11 access to the on board flash memory ceases. I have not yet had time to analyze this, just rolled back to 1.8.10 and functions again.

[rocketscream]

Same here and even if it worked with 1.8.10, it seems the bus clock has been set slower as default.

[facchinm]

Closing as duplicate of #593 , fixed by #622