net: phy: micrel: Staticize ksz8873mll_read_status()

ksz8873mll_read_status() is used only in this file.
Fix the following sparse warning:

drivers/net/phy/micrel.c:147:5: warning: symbol 'ksz8873mll_read_status' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>