msm: ipa4: fix the unclock gsi IPA register access

The while loop in gsi_handle_irq will cause an extra register
read after handle last interrupt. This read can be the unclocked
read. The fix is to take out the while loop.

Change-Id: If4b3bb127ee66648d24b8ca7c16bd6fec42d9fcc
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>