usb: pd: Always request maximum available current from PDO

Currently when selecting a PDO the request object is specifying
900mA as the operating current, which is suboptimal if the
supply is capable of more. Instead, simply request for the
maximum available. In the case where a PDO does not meet the
minimum current, set the capability mismatch flag and indicate
the required amount via the maximum field in the request.

Change-Id: I521059ab1c1d95df95fdede84512e73c5d9b2329
Signed-off-by: Jack Pham <jackp@codeaurora.org>