usb: core: phy: fix return value of usb_phy_roothub_exit()

usb_phy_roothub_exit() should return the error code from the phy_exit()
call if exiting the PHY failed.
However, since a wrong variable is used usb_phy_roothub_exit() currently
always returns 0, even if one of the phy_exit calls returned an error.
Clang also reports this bug:
kernel/drivers/usb/core/phy.c:114:8: warning: explicitly assigning value of
variable of type 'int' to itself [-Wself-assign] error, forbidden
warning: phy.c:114

Fix this by assigning the error code from phy_exit() to the "ret"
variable to propagate the error correctly.

Fixes: 07dbff0d ("usb: core: add a wrapper for the USB PHYs on the HCD")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>