Input: bcm5974 - prepare for a new trackpad header type

The new unibody Macbooks are equipped with an integrated button and
trackpad. The package header of the trackpad interface has changed to
also contain information about the integrated button.  This patch
performs the necessary preparations to allow for the new package
header.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>