net: dwc_eth_qos: Fix needless phy auto-negotiation restarts

Disabling clk_ck clock leads to link up status loss in phy, which
leads to auto-negotiation restart before each network command
execution.

This issue is especially big for PXE boot protocol because of
auto-negotiation restarts before each configuration filename trial.

To avoid this issue don't disable clk_ck clock after it was enabled.

Signed-off-by: Daniil Stas <daniil.stas@posteo.net>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>

Reviewed-by: Ramon Fried <rfried.dev@gmail.com>