r8152: fix the units of some registers for RTL8156A

The units of PLA_RX_FIFO_FULL and PLA_RX_FIFO_EMPTY are 16 bytes.

Fixes: 195aae32 ("r8152: support new chips")
Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>