iio:accel:stk8312: check for invalid value

Revision 1.2 of the datasheet recommends on page 22 to only write non-zero
values read from OTP register 0x70 into AFECTRL register.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Reviewed-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>