HID: intel-ish-hid: Remove unused variable 'err'

Fixes the following W=1 kernel build warning(s):

 drivers/hid/intel-ish-hid/ishtp/client.c: In function ‘ishtp_cl_disconnect’:
 drivers/hid/intel-ish-hid/ishtp/client.c:266:6: warning: variable ‘err’ set but not used [-Wunused-but-set-variable]

Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-input@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>