msm: ipa3: use proper amount of event rings

The number of event rings elements should correlate with the
number of transfer rings elements and both should be based on
desc_fifo_sz. Calculate the proper event ring length and allocate.

Change-Id: I5a786b33875eb70f0e49d237eec657473f6918fa
CRs-Fixed: 1097871
Signed-off-by: Amir Levy <alevy@codeaurora.org>