scripts/gdb: convert modules usage to lists functions

Simplify the module list functions with the new list_for_each_entry
abstractions

Link: http://lkml.kernel.org/r/ad0101c9391088608166fcec26af179868973d86.1462865983.git.jan.kiszka@siemens.com


Signed-off-by: Kieran Bingham <kieran.bingham@linaro.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>