msm: ipa: revert gsi check for erindex

Gsi_alloc_channel should complete successfully even if erindex is invalid
since for some channels even ring is not needed.

Change-Id: I41c20bb76daf701f261f8c956523b702b4b4c09f
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>