usb: r8a66597: Remove BE support

While the USB controller can work both in LE and BE modes, there is
no user for the BE mode, so drop it. If there ever is a user for it,
it can be easily re-added back.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Chris Brandt <chris.brandt@renesas.com>