spi: loopback-test: mark rx_ranges_cmp() static

We get 1 warning when building kernel with W=1:
drivers/spi/spi-loopback-test.c:408:5: warning: no previous prototype for 'rx_ranges_cmp' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>