staging: pi433: fix CamelCase for fifo variables

Fixes checkpatch warnings:

  CHECK: Avoid CamelCase: <fifoEmpty>
  CHECK: Avoid CamelCase: <fifoFillCondition>
  CHECK: Avoid CamelCase: <fifoFull>
  CHECK: Avoid CamelCase: <fifoLevel>
  CHECK: Avoid CamelCase: <fifoLevelBelowThreshold>
  CHECK: Avoid CamelCase: <fifoNotEmpty>
  CHECK: Avoid CamelCase: <fifoOverrun>

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