overlays: dwc2: Increase RX FIFO size

The previous version of the dwc2 overlay set the RX FIFO size to
256 4-byte words. This sounds large enough for a 1024 byte packet (the
largest isochronous high speed packet allowed), but it doesn't take
into account some extra space needed by the hardware.

Minas Harutyunyan at Synopsys (the source of the DWC OTG design)
came up with a more correct value, 301, but since there is spare packet
RAM this can be increased to 558 to allow two packets per frame.

See: https://github.com/raspberrypi/linux/issues/3447



Signed-off-by: Phil Elwell <phil@raspberrypi.com>