drivers: dwc_otg: use C11 style variable array declarations

The kernel C standard changed in 5.18.

Remove a layer of indirection around the FIQ bounce buffers, be consistent
with pointers to FIQ bounce buffers, and remove open-coded 32-bit clamping
of DMA addresses.

Also remove a pointless fiq_state initialisation loop.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>