mainboard/google/samus: Fix usage of GNU field designator ext

Following the reasoning in,
 8089f17 mainboard/lenovo/x230 Fix usage of GNU field designator extension

In C99 we defined a syntax for this. GCC's old syntax was deprecated.

Change-Id: Id3b16872f62660393d938d6f95977a4e3842d0d1
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7690
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>