drivers: uart: wch_usart: Fix typo in USART_WCH_IRQ_HANDLER

Previously, 'USART_WCH_IRQ_HANDLER' used 'DT_INST_IRQ(index, priority)',
which incorrectly referenced 'index' instead of 'idx'. This issue went
undetected because 'IRQ_CONNECT' drops the priority value on all boards
supported by this driver.

Fix by using 'DT_INST_IRQ(idx, priority)'.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>