net: bcmgenet: Remove checks on clock handles

Instead of multiplying the number of checks for IS_ERR(priv->clk),
simply NULLify the 'struct clk' pointer which is something the Linux
common clock framework perfectly deals with and does early return for
each and every single clk_* API functions.

Having every single function check for !IS_ERR(priv->clk) is both
redundant and error prone, as it turns out, we were doing it for the
main GENET clock: priv->clk, but not for the Wake-on-LAN or EEE clock,
so let's just be consistent here.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Petri Gynther <pgynther@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>