staging: iio: resolver: fix comparison to NULL

Remove comparison of spi->dev.platform_data to NULL by replacing it with
'!spi->dev.platform_data' as checkpatch suggested:

CHECK: Comparison to NULL could be written "!spi->dev.platform_data"

Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>