drivers: serial: sifive: Fix UART extern interrupts

IRQ_CONNECT and irq_enable calls in the SiFive UART driver were
misconfigured when the conversion to DeviceTree support occurred.

See also: https://github.com/zephyrproject-rtos/zephyr/pull/10613



Signed-off-by: Sören Tempel <soeren+git@soeren-tempel.net>