mtd: mark mtd_is_partition argument as constant

'struct mtd_info' is not modified by 'mtd_is_partition()' so it can be marked
as "const".

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>