hw/net/npcm_gmac.c: Correct test for when to reallocate packet buffer

hw/net/npcm_gmac.c: Correct test for when to reallocate packet buffer

In gmac_try_send_next_packet() we have code that does "if this block
of data won't fit in the buffer, reallocate it".  However, the
condition it uses is
  if ((prev_buf_size + tx_buf_len) > sizeof(buf))
where buf is a uint8_t *.

This means that sizeof(buf) is always 8 bytes, and the condition will
almost always be true, so we will reallocate the buffer more often
than we need to.

Correct the condition to test against tx_buffer_size, which is
where we track how big the allocated buffer is.

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