usb: mtu3: avoid TX data length truncated in SS/SSP mode

The variable of 'count' is declared as u8, this will cause an issue
due to value truncated when works in SS or SSP mode and data length
is greater than 255, so change it as u32.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>