staging: pi433: fix CamelCase for currentValue

Local variable storing the value for modulation register so replace
with modulation_reg.

Fixes checkpatch warnings:

  CHECK: Avoid CamelCase: <currentValue>

Signed-off-by: Valentin Vidic <Valentin.Vidic@CARNet.hr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>