staging:iio:naming in the EV_BIT macro fix.

Should always have been IIO_EV_DIR_MAX as that's what it actually controls.
Also reduced the number to 4 as not yet seen a case needing more and this
is not userspace visible anyway.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>