iwlwifi: make various things const

There are a number of things in the .data section that should
really be in .rodata, for example all ops structs and strings.
Mark everything const that can be, leaving the .data section
pretty much empty.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>