generic: platform/mikrotik: rb_hardconfig.c minor fixes

generic: platform/mikrotik: rb_hardconfig.c minor fixes

For the sake of strictly typed code, add a missing const qualifier.
Add a missing return value in error path.
Check the return value of mtd_read(), for good measure.
Also demote the error printks of failed sysfs file creation to warn
level since they are not fatal in the init() sequence.
Finally, add a note regarding PAGE_SIZE and clarify a comment.

Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Tested-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>