net: mvgbe: extract common code for SMI wait

Combine repeated code from smi_reg_read/smi_reg_write into a common
function smi_wait_ready.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>