[AVR] Don't worry about call frame size when initializing frame pointer

We previously only used the frame pointer if the frame pointer was too
big. This was to work around a bug (described in this old commit)

https://sourceforge.net/p/avr-llvm/code/204/tree//llvm/trunk/AVR/AVRFrameLowering.cpp?diff=50d64d912718465cb887d17a:203

I mistakenly invered the condition assuming it was a typo. I am now
removing it because it doesn't seem to be a problem anymore (plus it's a
dirty hack).

llvm-svn: 283639