gpio: mcp23s08: switch chip count to int

Commit 3e3bed91
"gpio: mcp23s08: fixed count variable for devicetree probing"
introduced a loop check to see if the number of chips were
unconsistent and going below zero counting downwards, but
this requires the counting variable to be able to be
negative, so switch the variable from unsigned to int.

Cc: Michael Stickel <ms@mycable.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>