HID: wacom: Simplify 'wacom_pl_irq'

Unlike other IRQ functions, 'wacom_pl_irq' uses the second element of
the 'tool' array to store information about its single pen. This makes
the function more difficult to understand (since it doesn't follow the
general pattern of other IRQ functions) and prevents the possibility of
refactoring how pen state is stored.

This patch rewrites 'wacom_pl_irq' to follow the usual IRQ conventions,
including storing tool type in 'tool[0]' and implicitly tracking prox
with the 'id[0]' variable.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>