zephy: Increase shell RX ring buf size
zephy: Increase shell RX ring buf size

FAFT tests send batched commands to the EC shell,
which were overflowing the 64 byte buffer.
Increase the RX buffer size to accommodate the FAFT requirements.
This was happening on both nivviks and nereid.

BUG=b:231646498
TEST=Run FAFT_Setup
BRANCH=none

Signed-off-by: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>
Change-Id: I5ead9e7413d3a3ce6072d4826c423adfed808e36
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3630104
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>

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