Increase NVIDIA overclock limit

[Anuskuss]

Personally I'm against this hand-holding (you already require root permissions to overclock at all) and think that the whole block of code gotta go but if you wanna keep this at least use some more realistic numbers. I can personally do 140/2790 (150 clock crashes, 2800 vram has artifacts) but I've seen people comfortably at 250 clock so 300 should be a good upper bound.
I would also suggest that instead of failing at all to just do a max(config_get_nv_core_clock_mhz_offset(), 300) but that would require a few more changes.

[afayaz-feral]

I agree that the hardcoded limit is unnecessary, and I assume the original author picked those specific values based on the hardware available at the time. Increasing the limit doesn't really solve the underlying problem.

I've been investigating the consequences of removing the limit altogether. I'll keep this PR open for visibility.