scripts/nsdeps: make sure to pass all module source files to spatch

The nsdeps script passes a list of the module source files to
generate_deps_for_ns() as a space delimited string named $mod_source_files,
which then passes it to spatch. But since $mod_source_files is not encased
in quotes, each source file in that string is treated as a separate shell
function argument (as $2, $3, $4, etc.).  However, the spatch invocation
only refers to $2, so only the first file out of $mod_source_files is
processed by spatch.

This causes problems (namely, the MODULE_IMPORT_NS() statement doesn't
get inserted) when a module is composed of many source files and the
"main" module file containing the MODULE_LICENSE() statement is not the
first file listed in $mod_source_files. Fix this by encasing
$mod_source_files in quotes so that the entirety of the string is
treated as a single argument and can be referred to as $2.

In addition, put quotes in the variable assignment of mod_source_files
to prevent any shell interpretation and field splitting.

Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>