msm: pcie: switch GPIO to sleep state before asserting PERST

Before asserting PERST, PCIe bus driver should switch GPIO to
sleep state. This will prevent host from missing any events after
the link is off.

Change-Id: Ie57d339da02dde6cbb3c41d2be52ee0602227193
Signed-off-by: Tony Truong <truong@codeaurora.org>