gst/gstpad.c: Use some more macros where it makes sense.

gst/gstpad.c: Use some more macros where it makes sense.

Original commit message from CVS:
* gst/gstpad.c: (gst_pad_init), (gst_pad_activate_pull),
(gst_pad_activate_push):
Use some more macros where it makes sense.
Allow pad mode switching instead of asserting. When a pad
is activated in one mode and we activate it in another,
deactivate it first before activating it in a different mode.
Fixes #329198.