usb: pd: Make select_pdo_store() wait until request goes through

When sending a request through the 'select_pdo' sysfs file,
make sure the request has gone through and is acknowledged
by the source before returning. This allows for checking if
the request was rejected so that an error can be returned.

Rename the 'swap_complete' completion variable to 'is_ready'
so it can be reused in this context. Also add a mutex so that
select_pdo will not contend with a dual_role swap.

Change-Id: Ie8b088faa15c67915e3bd96972b4f59f0cc66afe
Signed-off-by: Jack Pham <jackp@codeaurora.org>