net: bgmac: remove set but not used variable 'err'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/broadcom/bgmac.c: In function 'bgmac_dma_alloc':
drivers/net/ethernet/broadcom/bgmac.c:619:6: warning:
 variable 'err' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>