g: decorate _i2cs_write_complete_int() so that LTO does not loose it
g: decorate _i2cs_write_complete_int() so that LTO does not loose it

Otherwise LTO optimizes this function away before finding out that it,
in fact, needs this it.

BRANCH=cr50, cr50-mp
BUG=b:65253310
TEST=after the entire stack was applied Cr50 builds successfully with
     CONFIG_LTO enabled, and passes TCG test suite.

Change-Id: I1bd8292b7228efc982118e23455503fefd52c689
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1460087
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec