msm: ipa3: Validate IPA client mapping to endpoint index

ipa3_get_ep_mapping() is used to map an IPA client to
IPA endpoint index. The index is used as array index.
This change validates that the returned index is
within the valid range.

CRs-fixed: 2012203
Change-Id: I25df8d0c9b60a5290f2457dd2ed77f2c7e78b46f
Signed-off-by: Ghanim Fodi <gfodi@codeaurora.org>