bus: moxtet: fix unsigned comparison to less than zero

Currently the size_t variable res is being checked for
an error failure however the unsigned variable is never
less than zero so this test is always false. Fix this by
making variable res ssize_t

Link: https://lore.kernel.org/r/20190822010351.15660-2-marek.behun@nic.cz


Addresses-Coverity: ("Unsigned compared against 0")
Fixes: 5bc7f990 ("bus: Add support for Moxtet bus")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>