platform: gru: Fix sku_id return value

This should be an int, and it is an int. But for some reason I declared
the return value as 'char *'. Probably too much copy/paste from
get_version() or something...

BRANCH=scarlet
BUG=b:73506715
TEST=build, `mosys platform sku` on scarlet

Change-Id: Ia57f709ca031fbad9750ce75a64ff291f6fd08c1
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1033314
1 file changed