mtd: denali: fix the format of comment blocks

We should use
/*
 * Blah Blah ...
 * ...
 */

for multi-line comment blocks.

In addition, refactor some comments where it seems reasonable and
remove some comments where the code is clear enough such as:

    /* clear interrupts */
    clear_interrupts(denali);
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>