net: amd-xgbe: Toggle PLL settings during rate change

For each rate change command submission, the FW has to do a phy
power off sequence internally. For this to happen correctly, the
PLL re-initialization control setting has to be turned off before
sending mailbox commands and re-enabled once the command submission
is complete.

Without the PLL control setting, the link up takes longer time in a
fixed phy configuration.

Fixes: 47f164de

 ("amd-xgbe: Add PCI device support")
Co-developed-by: Sudheesh Mavila <sudheesh.mavila@amd.com>
Signed-off-by: Sudheesh Mavila <sudheesh.mavila@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>