staging:iio:adt7316 Fix some 'interesting' string operations

Calling memcmp() to check the value of the first byte in a string is overkill.
Just use buf[0] == '1' or buf[0] != '1' as appropriate.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>