Fix ucode CIC x105 detection after reset

After reset, IMEM could contain garbage which could interfere with ucode identification.
To avoid that, we reduce the length used to compute the ucode sum.

Furthermore, after fixing PIFBoot ROM emulation in mupen64plus-core, there is no need
for different values for CIC 6105 and CIC 7105.
Merge pull request #28 from bsmiles32/fix_x105_detection

Fix ucode CIC x105 detection after reset