pad: rework probe's hook_marshall function

pad: rework probe's hook_marshall function

PUSH and PULL mode have opposite scenarios for IDLE and BLOCK
probes.

For PUSH it will BLOCK with some data type and IDLE won't have a type.
For PULL it will BLOCK before getting some data and will be IDLE when
some data is obtained.

The check in hook_marshall was specific for PUSH mode and would cause
PULL probes to fail to be called. Adding different checks for the mode
to fix this issue.

https://bugzilla.gnome.org/show_bug.cgi?id=761211