net: dsa: rename switch operations structure

Now that the dsa_switch_driver structure contains only function pointers
as it is supposed to, rename it to the more appropriate dsa_switch_ops,
uniformly to any other operations structure in the kernel.

No functional changes here, basically just the result of something like:
s/dsa_switch_driver *drv/dsa_switch_ops *ops/g

However keep the {un,}register_switch_driver functions and their
dsa_switch_drivers list as is, since they represent the -- likely to be
deprecated soon -- legacy DSA registration framework.

In the meantime, also fix the following checks from checkpatch.pl to
make it happy with this patch:

    CHECK: Comparison to NULL could be written "!ops"
    #403: FILE: net/dsa/dsa.c:470:
    +	if (ops == NULL) {

    CHECK: Comparison to NULL could be written "ds->ops->get_strings"
    #773: FILE: net/dsa/slave.c:697:
    +		if (ds->ops->get_strings != NULL)

    CHECK: Comparison to NULL could be written "ds->ops->get_ethtool_stats"
    #824: FILE: net/dsa/slave.c:785:
    +	if (ds->ops->get_ethtool_stats != NULL)

    CHECK: Comparison to NULL could be written "ds->ops->get_sset_count"
    #835: FILE: net/dsa/slave.c:798:
    +		if (ds->ops->get_sset_count != NULL)

    total: 0 errors, 0 warnings, 4 checks, 784 lines checked

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