i2c-pxa2xx: Don't clear isr bits too early

isr is passed later into i2c_pxa_irq_txempty and
i2c_pxa_irq_rxfull and they may use some other bits
than irq sources.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>