When changing the state of the WR servo we can call a hook (function
pointer) to a function defined by a user in the upper layer (wrpc-sw).
The purpose of the hook is to add new functionalities when we reach a
specific state (Alarms, IRIGB, etc...) without needing to change the
code of PPSI.

[This is a rework of the original patch by Benoit. Mainly, I added
the extra integer argument to the hook (so I call it twice: both
entering and leaving the servo), and I removed the function to
set the hook from wrpc: the main function can set the pointer directly]
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>