CVE-2026-64234: tty: serial: pch_uart: add check for dma_alloc_coherent()

In the Linux kernel, the following vulnerability has been resolved:

tty: serial: pchuart: add check for dmaalloccoherent()

Add a check for dmaalloccoherent() failure to prevent a potential NULL pointer dereference in dmahandlerx(). Properly release DMA channels and the PCI device reference using a goto ladder if the allocation fails.