usb: pd: rename enum pd_msg_type to pd_sop_type

This enum is used to specify what packet type (SOP, SOP', SOP'')
to the PD PHY when sending and receiving messages. To avoid
confusion with the message type field of a message header (e.g.
MSG_REQUEST, MSG_ACCEPT, etc), rename the enum to pd_sop_type
and change the parameter name from 'type' to 'sop'. Also change
the enum pd_sig_type parameter name to 'sig' for similar clarity.
This patch is cosmetic, no functional changes.

Change-Id: Idf74f7635ec5afee3538815f2efd9bc032ee6d89
Signed-off-by: Jack Pham <jackp@codeaurora.org>