8250-men-mcb: fix error checking when get_num_ports returns -ENODEV

The current checking for failure on the number of ports fails when
-ENODEV is returned from the call to get_num_ports. Fix this by making
num_ports and loop counter i signed rather than unsigned ints. Also
add check for num_ports being less than zero to check for -ve error
returns.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: e2fea54e ("8250-men-mcb: add support for 16z025 and 16z057")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Michael Moese <mmoese@suse.de>
Link: https://lore.kernel.org/r/20191013220016.9369-1-colin.king@canonical.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>