iwlwifi: fix build error for IWLMEI

iwlwifi: fix build error for IWLMEI

When CONFIG_IWLWIFI=m and CONFIG_IWLMEI=y, the kernel build system
must be told to build the iwlwifi/ subdirectory for both IWLWIFI and
IWLMEI so that builds for both =y and =m are done.

This resolves an undefined reference build error:

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

Fixes: 977df8bd5844 ("wlwifi: work around reverse dependency on MEI")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Luca Coelho <luciano.coelho@intel.com>
Cc: linux-wireless@vger.kernel.org
Link: https://lore.kernel.org/r/20220227200051.7176-1-rdunlap@infradead.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>