Input: ff-memless - another fix for signed to unsigned overflow

The commit 9e68177e changed 'gain' from
signed to unsigned to fix an issue with rumble effect calculation, however
it introduced problems when calculating constant effects. Having 'gain'
being unsigned int was an unfortunate choice since it dominates all
implicit type conversions causing everything to be treated as unsigned
int.

Let's change it back to signed int and simply add proper casts to rumble
effect calculations.

Reported-by: Gary Stein <lordcnidarian@gmail.com>
Acked-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>