iio: ti_am335x_adc: fix static in function header

Static is missing in function header. Corrected.

Signed-off-by: Zubair Lutfullah <zubair.lutfullah@gmail.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>