i2c: i801: Add helper i801_get_block_len

Avoid code duplication and factor out retrieving and checking the
block length value to new helper i801_get_block_len().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>