drivers: net: cpsw: ethtool: fix accessing to suspended device

The CPSW might be suspended by RPM if all ethX interfaces are down,
but it still could be accesible through ethtool interfce. In this case
ethtool operations, requiring registers access, will cause L3 errors and
CPSW crash.

ethtool callbcaks which need to access CPSW registers now:
.set_coalesce(), .get_ethtool_stats(), .set_pauseparam(), .get_regs()

Hence, fix it by adding .begin()/.complete() ethtool callbacks, which
will be called before/after each ethtool operation runs, and do CPSW
RPM handling in these callbacks. That way CPSW will be active while
handling ethtool requests.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>