usb: pd: Fix compile warnings

Fix the following compiler warnings from gcc 6.1.1 with -Werror enabled:

 drivers/usb/pd/policy_engine.c:4274:
   format '%s' expects a matching 'char *' argument [-Werror=format=]
   usbpd_err(&pd->dev, "%s: Cannot send msg\n");

 drivers/usb/pd/policy_engine.c:4286:
   format '%s' expects a matching 'char *' argument [-Werror=format=]
   usbpd_err(&pd->dev, "%s: request timed out\n");

Fixes: 3f62e043 ("usb: pd: add snapshot of qpnp-pdphy and policy_engine drivers")
Change-Id: Ic0dedbad2da01c19793cb4aaf9a249ca68db1195
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>