oxnas: sata_oxnas: make irq handler more readable

safed one level of indention by using 'continue' instead of a
lengthy if-clause.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
v2: use logic-AND instead of '?' operator when checking for hw bug 6320

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43768 3c298f89-4303-0410-b956-a3cf2f4a3e73