mtd: use DEFINE_SHOW_ATTRIBUTE() instead of open-coding it

DEFINE_SHOW_ATTRIBUTE macro can help us simplify the code, so change
to it. And change the DEBUGFS_RO_ATTR macro defined in some file to a
standard macro.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>