drivers: net: phy: xgene: Fix 'remove' function

If 'IS_ERR(pdata->clk)' is true, then 'clk_disable_unprepare(pdata->clk)'
will do nothing.

It is likely that 'if (!IS_ERR(pdata->clk))' was expected here.
In fact, the test can even be removed because 'clk_disable_unprepare'
already handles such cases.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>