tests: driver: uart: fix unchecked return value

Coverity points out that uart_irq_update() return value
should be checked.

Coverity-CID: 166776

Change-Id: I3a754dae9e8f1563f4879e2fadfd89621785de8f
Signed-off-by: Qiu Peiyang <peiyangx.qiu@intel.com>
1 file changed