ipmi: kcs_bmc: Make status update atomic

Enable more efficient implementation of read-modify-write sequences.
Both device drivers for the KCS BMC stack use regmaps. The new callback
allows us to exploit regmap_update_bits().

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Zev Weiss <zweiss@equinix.com>
Message-Id: <20210608104757.582199-3-andrew@aj.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>