iio:bma180: Missing check for frequency fractional part

val2 should be zero

This will make no difference for correct inputs but will reject
incorrect ones with a decimal part in the value written to the sysfs
interface.

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org