net: dsa: b53: Use a macro to define I/O operations

Instead of repeating the same pattern: acquire mutex, read/write,
release mutex, define a macro: b53_build_op() which takes the type
(read|write), I/O size, and value (scalar or pointer). This helps with
fixing bugs that could exist (e.g: missing barrier, lock etc.).

Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>