tty: serial: bfin_sport_uart: fix signedness error

sport->port.irq is unsigned, check for <0 doesn't make sense.
Explicitly cast it to int to check for error.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>