iwlwifi: don't define OTP_LOW_IMAGE_SIZE per family, but per size

Using OTP_LOW_IMAGE_SIZE_FAMILY_8000/9000/22000 only obfuscates the
actual values, since these 3 are the same.  Redefine the values per
size so it's easier to understand and compare the different
configurations.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>