Fixed compiler warning in irq-generic.h

[Andreas Heinig]

Hello,

The following two patches remove one compiler warning in irq-genric.h.

Patch 1/2:
- removes the initial warning where the compiler complains about
  different signs while comparing the vector number

Patch 2/2:
- removes the new compiler warning generated after applying patch 1/2 if  
  BSP_INTERRUPT_VECTOR_MIN is set to 0. In this case we compare an
  unsigned integer for being lower than 0, which is obviously always true.

Regards,
Andreas