staging: pi433: Rename variable in struct pi433_rx_cfg

Renames variable thresholdDecrement in struct pi433_rx_cfg to
threshold_decrement to get rid of checkpatch.pl warning
"Avoid CamelCase: <thresholdDecrement>".

Signed-off-by: Simon Sandström <simon@nikanor.nu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>