mtd: spear_smi: release memory region during remove

Driver must cleanup all held resources during remove. It wasn't
releasing requested memory region.

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