MLK-12883 usb: limit USB_MAX_XFER_BLK to 256

For Some USB mass storage devices, such as:
"
 - Kingston DataTraveler 2.0 001D7D06CF09B04199C7B3EA
 - Class: (from Interface) Mass Storage
 - PacketSize: 64  Configurations: 1
 - Vendor: 0x0930  Product 0x6545 Version 1.16
"
When `usb read 0x80000000 0 0x2000`, we met
"EHCI timed out on TD - token=0x80008d80".

The devices does not support scsi VPD page, we are not able
to get the maximum transfer length for READ(10)/WRITE(10).

So we limit this to 256 blocks as READ(6).

Signed-off-by: Peng Fan <peng.fan@nxp.com>