modpost: refactor seen flag clearing in add_depends()

You do not need to iterate over all modules for resetting ->seen flag
because add_depends() is only interested in modules that export symbols
referenced from the given 'mod'.

This also avoids shadowing the 'modules' parameter of add_depends().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>