serial: mxs-auart: Remove unneeded zeroing of 'ret'

There is no need to explicitly zero the 'ret' variable as it is properly
initialized in a few lines below as:

ret = serial_mxs_probe_dt(s, pdev);

Remove the unneeded zeroing of 'ret'.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>