drm: bridge: cdns-mhdp8546: fix compile warning

On x64 we get:

drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c:751:10: warning: conversion from 'long unsigned int' to 'unsigned int' changes value from '18446744073709551613' to '4294967293' [-Woverflow]

The registers are 32 bit, so fix by casting to u32.

Fixes: fb43aa0a

 ("drm: bridge: Add support for Cadence MHDP8546 DPI/DP bridge")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Swapnil Jakhade <sjakhade@cadence.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200929091918.24813-1-tomi.valkeinen@ti.com