hw/net/npcm_gmac.c: Drop 'buf' local variable

hw/net/npcm_gmac.c: Drop 'buf' local variable

We use the local variable 'buf' only when we call dma_memory_read(),
and it is always set to &tx_send_buffer[prev_buf_size] immediately
before both of those calls.  So remove the variable and pass
tx_send_buffer + prev_buf_size to dma_memory_read().

This fixes in passing a place where we set buf = tx_send_buffer
but never used that value because we always updated buf to
something else later before using it.

Coverity: CID 1534027
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>