iio: chemical: sps30_i2c: fix buffer size in sps30_i2c_read_meas()

In the Linux kernel, the following vulnerability has been resolved: iio: chemical: sps30_i2c: fix buffer size in sps30_i2c_read_meas() sizeof(num) evaluates to sizeof(size_t) (8 bytes on 64-bit) instead of the intended __be32 element size (4 bytes). Use sizeof(*meas) to correctly match the buffer element type.