9p: saving negative to unsigned char

Saving -EINVAL as unsigned char truncates the high bits and changes it
into 234 instead of -22.  This breaks the test for "if (ret == -EINVAL)"
in parse_opts().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>