module.h: simplify MODULE_IMPORT_NS

In commit ca321ec7 ("module.h: allow #define strings to work with
MODULE_IMPORT_NS") I fixed up the MODULE_IMPORT_NS() macro to allow
defined strings to work with it.  Unfortunatly I did it in a two-stage
process, when it could just be done with the __stringify() macro as
pointed out by Masahiro Yamada.

Clean this up to only be one macro instead of two steps to achieve the
same end result.

Fixes: ca321ec7 ("module.h: allow #define strings to work with MODULE_IMPORT_NS")
Reported-by: Masahiro Yamada <masahiroy@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Matthias Maennich <maennich@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>