pci_dw: Drop unconditional explicit DEBUG define

This has been present since the first revision of the file. The debugf
macros have always been unused so it doesn't actually do anything
useful, and besides, debugging should not be unconditionally turned on
for a production driver. Moreover, this breaks the riscv LINT kernel
build as sys/conf/NOTES includes options DEBUG, resulting in a macro
redefinition error. This does not show up in the arm64 LINT kernel build
since that has an explicit nooptions DEBUG, which is dubious and should
be revisited. Rather than copy such a hack to riscv's NOTES, fix this
specific instance of DEBUG breaking.

Fixes: 896e217a0eae ("fu740_pci_dw: Add SiFive FU740 PCIe controller driver")
MFC after: 1 week