iio: magnetometer: ak8975: fix potential kernel stack memory leak

In the Linux kernel, the following vulnerability has been resolved: iio: magnetometer: ak8975: fix potential kernel stack memory leak Currently in the AK8975 driver there are four instances where potential uninitialized kernel stack memory leaks can occur. If i2c_smbus_read_i2c_block_data_or_emulated() returns a value less than the size of the buffer, uninitialized bytes are retained in the buffer and later the buffer is passed on to IIO buffers, potentially leaking memory to userspace. Fix this by adding checks whether the return value of the function is equal to the size of the buffer and subsequently if the value is lesser than zero to distinguish from a returned error code.