mtd: spear_smi: fix compilation warning

drivers/mtd/devices/spear_smi.c: In function 'spear_smi_probe':
drivers/mtd/devices/spear_smi.c:984:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>