trace: fsi: Print transfer size unsigned

Due to other bugs I observed a spurious -1 transfer size.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Alistair Popple <alistair@popple.id.au>
Link: https://lore.kernel.org/r/20191108051945.7109-5-joel@jms.id.au


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>