hw/net/npcm_gmac.c: Unify length and prev_buf_size variables

hw/net/npcm_gmac.c: Unify length and prev_buf_size variables

After the bug fix in the previous commit, the length and prev_buf_size
variables are identical, except that prev_buf_size is uint32_t and
length is uint16_t. We can therefore unify them. The only place where
the type makes a difference is that we will truncate the packet
at 64K when sending it; this commit preserves that behaviour
by using a local variable when doing the packet send.

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