net: miiphybb: fix casting error

Since the return value is a signed int, if the leading MSB of rdreg is a 1,
it will get signed extended and will return a negative value which is an
error even though we read the correct value.

Fixes: dfcc496e ("net: mii: Changes not made by spatch")
Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Acked-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>