staging: pi433: Move FIFO_THRESHOLD define to source file

FIFO_THRESHOLD is only being used in pi433_if.c source files.

Signed-off-by: Shahar Avidar <ikobh7@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240227221204.534131-6-ikobh7@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>