net: calxedaxgmac: read correct field in xgmac_desc_get_buf_len

xgmac_desc_get_buf_len appears to have a copy/paste error. flags is the
wrong field to read. We should be reading buf_size field. cpu_to_le32
should also be le32_to_cpu. This never really mattered as this function
is only used for DMA mapping calls which happen to be nops with coherent
DMA.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>