net: stmmac: Fix RX packet size > 8191

Ping problems with packets > 8191 as shown:

PING 192.168.1.99 (192.168.1.99) 8150(8178) bytes of data.
8158 bytes from 192.168.1.99: icmp_seq=1 ttl=64 time=0.669 ms
wrong data byte 8144 should be 0xd0 but was 0x0
16    10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
      20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
%< ---------------snip--------------------------------------
8112  b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf
      c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf
8144  0 0 0 0 d0 d1
      ^^^^^^^
Notice the 4 bytes of 0 before the expected byte of d0.

Databook notes that the RX buffer must be a multiple of 4/8/16
bytes [1].

Update the DMA Buffer size define to 8188 instead of 8192. Remove
the -1 from the RX buffer size allocations and use the new
DMA Buffer size directly.

[1] Synopsys DesignWare Cores Ethernet MAC Universal v3.70a
    [section 8.4.2 - Table 8-24]

Tested on SoCFPGA Stratix10 with ping sweep from 100 to 8300 byte packets.

Fixes: 286a8372 ("stmmac: add CHAINED descriptor mode support (V4)")
Suggested-by: Jose Abreu <jose.abreu@synopsys.com>
Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>