iwlwifi: enable experimental ucode support

ucode firmware may need to be released as experimental for testing or
debugging. released ucode filenames have the API version as the last
component.  experimental ucode files will have that component be "exp"
and the fw_version string reported by ethtool will also contain the
string EXP to clearly identify this ucode from released ucode.
EXP is short for EXPERIMENTAL since fw_version has a max lenght on 32.

this capability is controlled by Kconfig and defaulted to not be used.

Signed-off-by: Jay Sternberg <jay.e.sternberg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>