usb: renesas_usbhs: modify fifo clear timing

Pipe buffer should be cleaned before using it,
but should NOT be cleaned in pipe "prepare" function.
Because the pipe might be working in such timing.
This patch fixup this issue.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>