mtd: spear_smi: clear status register on init

It was observed that sometimes smi returned errors while resume from
suspend.

For safety reasons clear status register for any errors during init. In
absence of it smi can return failures during command transmissions.

Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>