libmtd: fix mtd_dev_present return value on legacy systems

libmtd: fix mtd_dev_present return value on legacy systems

On legacy systems, if "/proc/mtd" doesn't exist or gives a read error,
mtd_dev_present returns -1 (since it calls legacy_dev_present), contrary
to what's specified in the header file.

This causes checks like

  if (mtd_dev_present(n)) {
    ...
  }

to give false positives. Fix this by comparing the return value to 1.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>