iwlwifi: pcie: fix iwl_so_trans_cfg link error when CONFIG_IWLMVM is disabled

Randy reported an error on his randconfig builds:

ERROR: modpost: "iwl_so_trans_cfg" [drivers/net/wireless/intel/iwlwifi/iwlwifi.ko] undefined!

The problem was that when CONFIG_IWLMVM was disabled we were still accessing
iwl_so_trans_cfg. Fix it by moving IS_ENABLED() check before the access.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: 930be4e7

 ("iwlwifi: add support for SnJ with Jf devices")
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Acked-by: Luca Coelho <luciano.coelho@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1614236661-20274-1-git-send-email-kvalo@codeaurora.org