serial: pic32_uart: Fix 'request_irq' and 'free_irq' inconsistancy

'request_irq' and 'free_irq' should have the same 'dev_id'.
Here one uses 'port', and the other one uses 'sport'.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>