ar71xx: fix potential IRQ misses during dispatch for qca953x

If both interrupts are set in the current implementation
only the 1st will be handled and the 2nd will be skipped
due to the "if else" condition.

Fix this by using the same approach as done for QCA955x
just below it.

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>