Currently the user can set ff_effects_max to zero with the EV_FF bit (and
the FF_GAIN and/or FF_AUTOCENTER bits) set, in this case the uninitialized
methods ff->set_gain and/or ff->set_autocenter can be dereferenced,
resulting in a kernel oops.

Check in uinput_create_device() and print a helpful message and return
-EINVAL in case the check fails.
Signed-off-by: Elias Vanderstuyft <elias.vds@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>