HID: wacom: Remove static WACOM_PKGLEN_MAX limit

Rather than memcpy every packet that we receive from HID into our own
local fixed-size array, we can just access the data directly through
the original pointer. While we're at it, remove the few other places
that assume a fixed maximum packet size and make them dynamic (in
particular: temporary buffers created by the wacom_wac_queue_flush and
wacom_intuos_bt_process_data functions; and the pen_fifo FIFO).

To ensure pen_fifo allocation has access to the maximum used packet
length, this commit also moves the function call to occur a little
later in the probe process.

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