msm: ipa: fix KW errors of access to invalid array element

This change adds checks to the returned value from
ipa_get_ep_mapping on the caller side in order to avoid
accessing the relevant structure out of bounds.

Change-Id: I2ef8563d93afe3682b3092a5ecf611ba370528fb
Acked-by: David Arinzon <darinzon@qti.qualcomm.com>
Signed-off-by: Sivan Reinstein <sivanr@codeaurora.org>