iwlwifi: fix the gcc build

• Only apply the previously added CWARNFLAGS to drv.c instead of the whole module.
• Only apply -Wno-initializer-overrides to CWARNFLAGS in the clang scenario as it's not supported with gcc.

This fixes building the module with gcc and avoids accidentally
introducing tech debt with the module, in the event other issues are
accidentally introduced.

MFC after: 3 days
Fixes: 6b627f8858 ("iwlwifi: update Intel's mvm/mld drivers")
Differential Revision: https://reviews.freebsd.org/D53591