Fix the following sparse warning:

./spidev_test.c:50:9: warning: symbol 'default_tx' was not declared. Should it be static?
./spidev_test.c:59:9: warning: symbol 'default_rx' was not declared. Should it be static?
./spidev_test.c:60:6: warning: symbol 'input_tx' was not declared. Should it be static?
Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Link: https://lore.kernel.org/r/1591880212-13479-1-git-send-email-zhangqing@loongson.cnSigned-off-by: Mark Brown <broonie@kernel.org>