spi: spidev_test: Use perror() only if errno is not 0

It is better to use perror() only if errno is not 0, it should use printf()
when errno is 0, otherwise there exists redudant ": Success".

E.g. without this patch:

$ ./spidev_test -p 1234 --input test.bin
only one of -p and --input may be selected: Success
Aborted (core dumped)

With this patch:

$ ./spidev_test -p 1234 --input test.bin
only one of -p and --input may be selected
Aborted (core dumped)

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1581567368-8055-3-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Mark Brown <broonie@kernel.org>