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 <[email protected]>
Tested-by: Roger Pueyo Centelles <[email protected]>
Signed-off-by: Thibaut VARÈNE <[email protected]>