dmaengine: sirf: fix irq number error check

irq_of_parse_and_map() returns 0 on error, no NO_IRQ, so the
failure condition can never be met.

This changes the comparison to check for zero instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>