Fix triple-finger taps reported as double-finger for Elan hw v.4 touchpads

Wait for all advertised head packets after status packet have been received.
This fixes rare but quite annoying issue in Elan hw v.4 touchpads support
when triple-finger taps are reported as double-finger taps under several
circumstances.

Reviewed by: gonzo
Approved by: gonzo (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D10266