atl1c: change atl1c_buffer struct and restructure clean atl1c_buffer procedure

change atl1c_buffer struct, use "u16 flags" instead of "u16 state"
to store more infomation for atl1c_buffer, and restructure clean
atl1c_buffer procedure, add common api atl1c_clean_buffer.

Signed-off-by: Jie Yang <jie.yang@atheros.com>
Signed-off-by: David S. Miller <davem@davemloft.net>