Doxygen: fix documentation of osmo_timer_setup()

This change fixes the following Doxygen warnings:

  timer.c:69: warning: argument 'callback' of command @param is not
              found in the argument list of
              osmo_timer_setup(struct osmo_timer_list *timer,
                               void(*cb)(void *data), void *data)
  timer.c:69: warning: argument 'pointer' of command @param is not
              found in the argument list of
              osmo_timer_setup(struct osmo_timer_list *timer,
                               void(*cb)(void *data), void *data)
  core/timer.h:70: warning: The following parameters of
                   osmo_timer_setup(struct osmo_timer_list *timer,
                   void(*cb)(void *data), void *data)
                   are not documented:
                     parameter 'cb'
                     parameter 'data'

Change-Id: If5668f40a7bfde2f4f22329a071c8c6eff23b99e
diff --git a/src/timer.c b/src/timer.c
index 02160a1..0b2e3dd 100644
--- a/src/timer.c
+++ b/src/timer.c
@@ -68,8 +68,8 @@
 
 /*! set up timer callback and data
  *  \param[in] timer the timer that should be added
- *  \param[in] callback function to be called when timer expires
- *  \param[in] pointer to data that passed to the callback function
+ *  \param[in] cb function to be called when timer expires
+ *  \param[in] data pointer to data that passed to the callback function
  */
 void osmo_timer_setup(struct osmo_timer_list *timer, void (*cb)(void *data),
 		      void *data)