Staging: iio: accel: sca3000: Fixed NULL comparison style

The variable u8 **rx_p, is a pointer-to-pointer and hence the check
should
be "if (!*rx_p)" and not "if (!rx_p)".
In the earlier version, checkpatch.pl gave the following check, which
was incorrect:
CHECK: Comparison to NULL could be written "!rx_p"
+       if (*rx_p == NULL) {

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>