drivers: serial: cmsdk_apb: fix irq_rx_ready deviation

The docstring for uart_cmsdk_apb_irq_rx_ready says "@return 1 if an
interrupt is ready, 0 otherwise" but the function actually returns 2 on
success. This commit fixes this to conform with serial driver API.

Signed-off-by: Jason Mitchell <jmitchell@radicalsemiconductor.com>