cadence_gem: avoid stack-writing buffer-overrun

Use sizeof(rxbuf)-size (not sizeof(rxbuf-size)) as the number
of bytes to clear.  The latter would always clear 4 or 8
bytes, possibly writing beyond the end of that stack buffer.
Alternatively, depending on the value of the "size" parameter,
it could fail to initialize the end of "rxbuf".
Spotted by coverity.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Reviewed-by: Peter A.G. Crosthwaite <peter.crosthwaite@petalogix.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
