msm: ipa: switch to interrupt mode only when needed

Do not change sps config in case IPA driver is polling mode as this
indicates that sps driver already notified EOT event and sps config
should not change until ipa driver processes the packet.

Change-Id: I28a6980c7cab2a5e7cf75817287dd190efcad9be
CRs-Fixed: 954307
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>