cxgb4/cxgb4vf: For T6 adapter, set FBMIN to 64 bytes

T4 and T5 hardware will not coalesce Free List PCI-E Fetch Requests if
the Host Driver provides more Free List Pointers than the Fetch Burst
Minimum value.  So if we set FBMIN to 64 bytes and the Host Driver
supplies 128 bytes of Free List Pointer data, the hardware will issue two
64-byte PCI-E Fetch Requests rather than a single coallesced 128-byte
Fetch Request. T6 fixes this. So, for T4/T5 we set the FBMIN value to 128

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>