Input: atmel_mxt_ts - don't double-unregister input dev on rmmod

Both mxt_free() and mxt_free_object_table() unregister the input device
unconditionally, yet mxt_free() calls mxt_free_object_table(). This
results in a double-unregister, and associated kernel crashes. Remove the
duplicate call from mxt_free(). Also, guard the unregister call in
mxt_free_object_table() so that if the input device was never created and
registered at all, no attempt is made to unregister it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>