staging:iio:magnetometer:ak8975 use standard i2c_smbus read functions.

Now the mysterious NOSTART flag is gone from the read, we can use the
i2c_smbus_read_byte/word/i2c_block_data functions instead of the
local reimplementation of these standard functions.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>