HID: wacom: Properly free inputs if 'wacom_allocate_inputs' fails

The 'wacom_allocate_inputs' function tries to allocate three input devices: one
each for the pen, touch, and pad. The pointers that are returned by the
'wacom_allocate_input' calls are temporarily stored to local variables where
they are checked to ensure they're non-null before storing them in the
'wacom_wac' structure. If an allocation fails,  the 'wacom_free_inputs'
function is called to reclaim the memory. Unfortunately, 'wacom_free_inputs' is
called prior to the pointers being copied, so it is not actually able to free
anything.

This patch has the calls to 'wacom_allocate_input' store the pointer directly
in the 'wacom_wac' structure where they can be freed. Also, it replaces the
call to 'wacom_free_inputs' with the (more general) 'wacom_clean_inputs' and
removes the no-longer-used function.

[jkosina@suse.com: modify to resolve conflict with 67e123ff ("Delete
 unnecessary checks")]

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