net: bgmac: Fix endian access in bgmac_dma_tx_ring_free()

bgmac_dma_tx_ring_free() assigns the ctl1 word which is a litle endian
32-bit word without using proper accessors, fix this, and because a
length cannot be negative, use unsigned int while at it.

Fixes: 9cde9450 ("bgmac: implement scatter/gather support")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>