serial: 8250_mid: Fix NULL function pointer dereference on DNV/ICX-D/SNR platforms

In the Linux kernel, the following vulnerability has been resolved: serial: 8250_mid: Fix NULL function pointer dereference on DNV/ICX-D/SNR platforms Commit b1b4efea05a5 ("serial: 8250_mid: Disable DMA for selected platforms") replaced the dnv_board setup and exit callbacks with PTR_IF(false, ...), which evaluates to NULL. However, the three call sites in mid8250_probe() and mid8250_remove() unconditionally dereference these function pointers without NULL checks, causing a NULL pointer dereference (kernel oops) on any Denverton (DNV), Ice Lake Xeon D (ICX-D/CDF), or Snowridge (SNR) platform. Fix this by adding the missing NULL checks before calling the setup and exit callbacks.