bug #36410: avr/boot.h: poisoned SPMCR for ATmega128

Don't evaluate SPMCR if SPMCSR was already around.



git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2403 1084037a-409f-4a99-8160-1a4b3577d4c1