usb: storage: Fix overwritten in usb_stor_set_max_xfer_blk()

The stored 'blk' value is overwritten to 'size / 512' before it can
be used in usb_stor_set_max_xfer_blk(). This is not what we want.
In fact, when 'size' exceeds the upper limit (USHRT_MAX * 512), we
should simply assign 'size' to the upper limit.

Reported-by: Coverity (CID: 167250)
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>