Correct CORTEX_MAX_KERNEL_PRIORITY use
Correct CORTEX_MAX_KERNEL_PRIORITY use

Now the NVIC minidriver always take the priority as parameter, the use
of the CORTEX_PRIORITY_MASK() macro is no more required.

Reference: http://www.chibios.org/dokuwiki/doku.php?id=chibios:articles:porting_from_2_to_3

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