[ARM] 5250/1: unbalanced enable_irq() for serial_ks8695.c fix

The function ks8695uart_set_termios() would cause an unbalanced
enable_irq() generated message to be printk()ed. This is because
there was no book keeping support to remember if the calls to
enable_irq() and disable_irq() were balanced for the modem
control irq.

Signed-off-by: Dick Hollenbeck <dick@softplc.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>