iwlwifi: traverse linklist to find the valid OTP block

iwlwifi: traverse linklist to find the valid OTP block

For devices using OTP memory, EEPROM image can start from
any one of the OTP blocks. If shadow RAM is disabled, we need to
traverse link list to find the last valid block, then start the EEPROM
image reading.

If OTP is not full, the valid block is the block _before_ the last block
on the link list; the last block on the link list is the empty block
ready for next OTP refresh/update.

If OTP is full, then the last block is the valid block to be used for
configure the device.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>