msm: ipa: Validate pdn_idx on modify_flt_rule

Replace the incomplete validate code on ipa_mdfy_flt_rule with
the already present ipa_validate function which is checks for
pdn_idx and is more complete.

Change-Id: Icfbd0c24ee832ce7c7a7af079219cbdf0c455b60
Crs-fixed: 2250028
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>