net: fec: Remove bogus flush_dcache_range() call

Remove incorrectly called and duplicate flush_dcache_range() call
from fec_mxc driver. The call is not needed, since the caches are
already flushed in fec_tbd_init(), moreover the second argument should
be the ending address, not size.

Signed-off-by: Marek Vasut <marex@denx.de>
Reported-by: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>