usb: gadget: composite: Fix bMaxPower for SuperSpeedPlus

SuperSpeedPlus peripherals must report their bMaxPower of the
configuration descriptor in units of 8mA as per the USB 3.2
specification. The current switch statement in encode_bMaxPower()
only checks for USB_SPEED_SUPER but not USB_SPEED_SUPER_PLUS so
the latter falls back to USB 2.0 encoding which uses 2mA units.
Replace the switch with a simple if/else.

Fixes: eae5820b ("usb: gadget: composite: Write SuperSpeedPlus config descriptors")
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>