net: stmmac: visconti: convert to set_clk_tx_rate() method

Convert visconti to use the set_clk_tx_rate() method. By doing so,
the GMAC control register will already have been updated (unlike with
the fix_mac_speed() method) so this code can be removed while porting
to the set_clk_tx_rate() method.

There is also no need for the spinlock, and has never been - neither
fix_mac_speed() nor set_clk_tx_rate() can be called by more than one
thread at a time, so the lock does nothing useful.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1u5SiQ-001I0B-OQ@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>