usb: pd: Only request current amount for explicit contract

The policy engine should only request for a specific current
amount when establishing an explicit contract. When performing
a hard reset, suspend this by requesting 0 and only if it
was non-zero before, otherwise requesting the maximum may
invalidate the default current for non-PD cases.

Change-Id: Icbef00d61f8705429105f241161362ad9f1f9d30
Signed-off-by: Jack Pham <jackp@codeaurora.org>