net: stmmac: dwmac-visconti: Fix value of ETHER_CLK_SEL_FREQ_SEL_2P5M

ETHER_CLK_SEL_FREQ_SEL_2P5M is not 0 bit of the register. This is a
value, which is 0. Fix from BIT(0) to 0.

Reported-by: Yuji Ishikawa <yuji2.ishikawa@toshiba.co.jp>
Fixes: b38dd98f

 ("net: stmmac: Add Toshiba Visconti SoCs glue driver")
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Link: https://lore.kernel.org/r/20211223073633.101306-1-nobuhiro1.iwamatsu@toshiba.co.jp
Signed-off-by: Jakub Kicinski <kuba@kernel.org>