Input: sur40 - validate number of endpoints before using them

Make sure to check the number of endpoints to avoid dereferencing a
NULL-pointer or accessing memory that lie beyond the end of the endpoint
array should a malicious device lack the expected endpoints.

Fixes: bdb5c57f

 ("Input: add sur40 driver for Samsung SUR40... ")
Signed-off-by: Johan Hovold <johan@kernel.org>
Cc: stable@vger.kernel.org	# 3.13
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>