usb: storage: Refactor to use max_xfer_blk from struct us_data

This adds a new memeber max_xfer_blk in struct us_data to record
the maximum number of transfer blocks for the storage device.

It is set per HCD setting, and so far is to 65535 for EHCD and 20
for everything else.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>