serial: samsung: fix leaked requested irq

The s3c64xx_serial_startup() requests an IRQ, but the common
s3c24xx_serial_shutdown() would not free it.

s3c64xx_serial_startup() is used when
s3c24xx_serial_has_interrupt_mask(port) is true, at which point
  ourport->rx_claimed = ourport->tx_claimed = 1
so we only need to free in one of the claimed cases.

Change-Id: I8e17ce52cf48dbf3faa26ac24c8a71f016771f32
Signed-off-by: JP Abgrall <jpa@google.com>