m68k: coldfire: make mcf_maskimr() static

When building with W=1:

      CC      arch/m68k/coldfire/intc.o
    arch/m68k/coldfire/intc.c:83:6: warning: no previous prototype for ‘mcf_maskimr’ [-Wmissing-prototypes]
     void mcf_maskimr(unsigned int mask)
          ^~~~~~~~~~~

The mcf_maskimr() function is only used within this file, make it static
to reduce name space pollution and fix warning.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>