docbook: change iomap source filename in deviceiobook

Fix warning since kernel-doc comments were moved to another
source file.

Warning(lib/iomap.c): no structured comments found

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 file changed