spi/documentation: Fix usage of __initdata

__initdata should be placed between the variable name and equal
sign for the variable to be placed in the intended section. Fix
the example code.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>