iwlwifi: mvm: don't allow A band if SKU forbids it

The driver wasn't reading the NVM properly. While this
didn't lead to any issue until now, it seems that there
is an old version of the NVM in the wild.
In this version, the A band channels appear to be valid
but the SKU capabilities (another field of the NVM) says
that A band isn't supported at all.
With this specific version of the NVM, the driver would
think that A band is supported while the HW / firmware
don't. This leads to asserts.

Cc: <stable@vger.kernel.org> [3.10+]
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>