dscc4: Undefined signed int shift

My analysis in the below mail applies, although the second part is
unnecessary because i isn't used in arithmetic operations here:

https://marc.info/?l=openbsd-tech&m=145377854103866&w=2



Thanks for your time.

Signed-off-by: Michael McConville <mmcco@mykolab.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>