usb: dwc2: make casts of ep->dma_buf consistent

In most places in the code we cast this to an unsigned long, but in one
place we cast to an unsigned int.  For consistency and to fix a warning
on 64bit targets, always cast this to unsigned long.  For the long term
we should however change the declaration of dma_buf.

Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>