ARM: imx8m: fix imx_eqos_txclk_set_rate() type mismatch for LTO

When building imx8mp_evk_defconfig with LTO, the compiler complains
about type mismatch of function imx_eqos_txclk_set_rate() in file
  drivers/net/dwc_eth_qos.c:845:12
which contains a weak definition of this function, vs file
  arch/arm/mach-imx/imx8m/clock_imx8mm.c
which contains an implementation.

Change the type of this function in the implementation to fix this.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>