g: fix sps interrupt assertion logic
g: fix sps interrupt assertion logic

The SPI initialization interrupt needs to be generated only when there
was actual data received while CS was asserted and after transaction
finished (i.e. CS is de-asserted).

BRANCH=cr50
BUG=b:35774896
TEST=verified on a bob with updated AP firmware

Change-Id: Ifc4b11870d511d47e9607a2001d845ee1e153f7f
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/452792
Reviewed-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>

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