msm: ipa: fix the return value in ipa_get_ep_mapping

The value of -1 is considered as an "unmapped" client ID to a pipe
in IPA. Even in the case of an invalid client ID, the above needs
to be returned, as it basically has the same meaning.

Change-Id: I9c2898ce9e1476a89cd866137d66f5048b5feff3
Acked-by: David Arinzon <darinzon@qti.qualcomm.com>
Signed-off-by: Sivan Reinstein <sivanr@codeaurora.org>