[chirp_devel] [tk760.py] Increase OEM band edges by 4.0%, fix #4789

Some of this model's variants are in ranges near ham bands but outside them by the OEM stated range, for example from 150 to 174 Mhz, or 148 to 174 Mhz, in the case of the UHF we has the variants in the 450 to 490 Mhz or the 406 to 430 Mhz.

I make some characterizations and most of the radios can work safely with a 4% of the stated range safely and will allow to work near ham bands. (real range is bigger but we stick to a safe value)

This patch was based on the discussions about issue #4709 and improvements from the chirp_devel mailing list comments.