net: dsa: b53: Fix sparse warnings in b53_mmap.c

sparse complains about the following warnings:

drivers/net/dsa/b53/b53_mmap.c:33:31: warning: incorrect type in
initializer (different address spaces)
drivers/net/dsa/b53/b53_mmap.c:33:31:    expected unsigned char
[noderef] [usertype] <asn:2>*regs
drivers/net/dsa/b53/b53_mmap.c:33:31:    got void *priv

and indeed, while what we are doing is functional, we are dereferencing
a void * pointer into a void __iomem * which is not great. Just use the
defined b53_mmap_priv structure which holds our register base and use
that.

Fixes: 967dd82f ("net: dsa: b53: Add support for Broadcom RoboSwitch")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>