msm: gsi: Avoid unnessary write to ipa

Only write to ipa to reset channel interrupt, when there
is no entry. Do a read of read pointer after write to close
a small window. The sequence is as such in the poll channel:
 If sw needs to update sw read pointer, it does,
  step 1. update read pointer from ipa,
  step 2  if there is no event, write to reset channel
    interrupt.
  step 3, to close window between step 1 and step2 that
    may have new events, do another read of read pointer.
    And if no more entry then, return EMPTY.
    Otherwise proceeed to process entries as before.

Change-Id: I68b983111416b617b835de024a22c47517ea7f3e
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>