[media] imon: fix Knob event interpretation issues on ARM

Events for the iMon Knob pad where not correctly interpreted on ARM,
resulting in buggy mouse movements (cursor going straight out of the
screen), key pad only generating KEY_RIGHT and KEY_DOWN events.
A reproducer is:
int main(int argc, char ** argv)
{
        char rel_x = 0x00; printf("rel_x:%d @%s:%d\n", rel_x, __FILE__, __LINE__);
        rel_x = 0x0f; printf("rel_x:%d @%s:%d\n", rel_x, __FILE__, __LINE__);
        rel_x |= ~0x0f; printf("rel_x:%d @%s:%d\n", rel_x, __FILE__, __LINE__);
        return 0;
}
(running on x86 or amd64)
$ ./test
rel_x:0 @test.c:6
rel_x:15 @test.c:7
rel_x:-1 @test.c:8
(running on armv6)
rel_x:0 @test.c:6
rel_x:15 @test.c:7
rel_x:255 @test.c:8
Forcing the rel_x and rel_y variables as signed char fixes the issue.

Reference: http://www.arm.linux.org.uk/docs/faqs/signedchar.php

Signed-off-by: Alexandre Lissy <alexandrelissy@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>