Fix frequency verb invalid frequency behavior

The frequency verb was not checking if a frequency corresponds to the
frequency band step the binding provides to users, add logic to do so
and return an error if the frequency is invalid.  While this is a
change in behavior, it is believe it should not impact users, as the
binding is intended to reflect user facing radio application usage.

Additional tweaks:
- fix error invalid frequency error message in reply JSON.
- add the same band limit checks to the null implementation's
  frequency setting function that the other implementations do.
- fix a couple of issues in README.md; the "rds" verb was missing,
  and the value key name for the "frequency" event was incorrect.

Bug-AGL: SPEC-3620

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I2adfc10448546264110f6d854f02d677087f1e47

0 files changed, 0 insertions, 0 deletions