npcx: shi: Enable SHI interrupt from CS interrupt
npcx: shi: Enable SHI interrupt from CS interrupt

Enable the SHI interrupt only after we have received and begun
processing our host command. Disable the SHI interrupt once our
transaction is complete (with either success or error status). This will
prevent the SHI interrupt from being asserted at the same time as the CS
interrupt, which can lead to the SHI interrupt being serviced first.
Also, it avoids needless, non-useful SHI interrupts during error
transactions.

BUG=chrome-os-partner:55710,chrome-os-partner:55795,chrome-os-partner:56254
BRANCH=None
TEST=Manual on gru. Stress test flashrom w/ unpowered Donette attached
(for host command spam), verify no errors encountered after 100 minutes.
Also verify host command interface functions properly after sysjump.

Signed-off-by: Shawn Nematbakhsh <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/384583
Reviewed-by: Mulin Chao <mlchao@nuvoton.com>
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>
(cherry picked from commit 668763721aab31a102ead348d7cfe1c72f50cb71)

Change-Id: I7292d1b34d3e1c71628a8b5d663ce9fd865493f4
Reviewed-on: https://chromium-review.googlesource.com/386447
Commit-Ready: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Tested-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Reviewed-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>

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