staging: pi433: fix frequency deviation check

staging: pi433: fix frequency deviation check

rf69 datasheet states that frequency deviation must exceed 600 Hz but
also that frequency deviation + (bitrate / 2) should be less than equal
to 500 kHz to ensure proper modulation.

This patch validates that both conditions are met so RF intersymbol
interference is less likely to happen due to misconfiguration of the uC

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Link: https://lore.kernel.org/r/20220103222334.GA6814@mail.google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>