libmtd: perform device checking first

If we don't check for the MTD before calling `legacy_get_dev_info1', we may
get errors like:

libmtd: MTD subsystem is old and does not support sysfs, so MTD character device nodes have to exist
libmtd: error!: "/dev/mtd2" is not a character device
mtdinfo: error!: libmtd failed get MTD device 2 information
         error 22 (Invalid argument)

So reverse the order of these two checks.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>