iwlwifi: fix incorrect otp blocks number for 6x50 series

For 6x50 series, number of OTP blocks is different from 6x00 series

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>