HID: steam: Prevent NULL pointer dereference in steam_{recv,send}_report

It is possible for a malicious device to forgo submitting a Feature
Report.  The HID Steam driver presently makes no prevision for this
and de-references the 'struct hid_report' pointer obtained from the
HID devices without first checking its validity.  Let's change that.

Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-input@vger.kernel.org
Fixes: c164d6ab

 ("HID: add driver for Valve Steam Controller")
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>