drivers/block/mg_disk.c: make mg_times_out() static

mg_times_out() is used only in this file.  Fix the following sparse
warning:

  drivers/block/mg_disk.c:639:6: warning: symbol 'mg_times_out' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>