arch/arm/samv7: fix sporadic bit flip after internal flash byte write

[pkarashchenko]

Summary

Fix sporadic bit flip after internal flash byte write

The number of embedded flash wait states was calculated in a wrong way and the code to set flash wait states was executed three times instead of one. Two calls are removed now and only in sam_clockconfig() remains

Impact

Improve progmem write for SAMv7 based devices

Testing

Verified with custom SAMe70 based board