iwlwifi: fix modular 3945 only build

If only 3945 is selected, and is a module, build
fails because iwl-legacy.c won't be compiled. Fix
this by adding it to the build correctly.

This doesn't happen for 4965 because it is a bool
option, not tristate, since it's built into the
AGN module.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Tested-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>