Somehow this driver has never had valid_bands defined?!

Fixes #11386

Right now we will choose the tuning step for a new edit from the
bandplan, even if that doesn't work for the frequency entered.
Ironically, the most common frequency in the NA bandplan (146.520)
would choose a step of 25kHz, which isn't actually aligned. Radios
that really care about this (i.e. kenwood live radios) will balk at
146.52 at 25kHz, allow it at 20kHz, and allow 146.50 at 25kHz.

This makes memedit use the band plan step if it is valid for the
entered frequency, else fall back to the minimum-viable logic that
we use when there is no bandplan step.

Fixes #11391