board/mpl/common/isa.c: Fix GCC 4.6 build warnings

Fix:
../common/isa.c: In function 'handle_isa_int':
../common/isa.c:385:21: warning: variable 'isr2' set but not used
[-Wunused-but-set-variable]
../common/isa.c:385:16: warning: variable 'isr1' set but not used
[-Wunused-but-set-variable]

Signed-off-by: David Mueller <d.mueller@elsoft.ch>