cr50: reduce UART TX buffer size
cr50: reduce UART TX buffer size

As we are getting short on SRAM and do not need that extensive UART
debugging support, let's reduce the UART TX buffer size to a standard
value of 512 bytes.

BRANCH=none
BUG=chrome-os-partner:58961
TEST=built and booted a new image. Verified that the cr50 console
     connection is still operational.

Change-Id: Ieb77671cb144b9ea5073f5d3242c9e41e5d78883
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/418948
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