usb: pd: allow outgoing requests after receiving status payload

Rearrange handling of received messages in PE_SNK_READY state
such that certain status payloads (Source_Cap_Ext, Status,
PPS_Status, et al) do not terminate. This allows pending request
messages to be handled in the same context and sent out before
exiting the usbpd_sm() worker, avoiding a need to reschedule.
This fixes an issue where an Alert message is received triggering
a Get_Status request to be sent but causes a pending power
Request to be preempted and never scheduled to be sent again.

Change-Id: I1c2b7f126e24a93095028c93b210d89b514183d0
Signed-off-by: Jack Pham <jackp@codeaurora.org>