mtd: mtdchar: return expected errors on mmap() call

According both to POSIX.1-2008 and Linux Programmer's Manual mmap()
syscall shouldn't return undocumented ENOSYS, this change replaces
the errno with more appropriate ENODEV and EACCESS.

Signed-off-by: Vladimir Zapolskiy <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

drivers/mtd/mtdchar.c

···
 		return (unsigned long) -EINVAL;
 
 	ret = mtd_get_unmapped_area(mtd, len, offset, flags);
-	return ret == -EOPNOTSUPP ? -ENOSYS : ret;
+	return ret == -EOPNOTSUPP ? -ENODEV : ret;
 }
 #endif
 
···
 #endif
 		return vm_iomap_memory(vma, map->phys, map->size);
 	}
-	return -ENOSYS;
+	return -ENODEV;
 #else
-	return vma->vm_flags & VM_SHARED ? 0 : -ENOSYS;
+	return vma->vm_flags & VM_SHARED ? 0 : -EACCES;
 #endif
 }