parisc: Use signed char for hardware path in pdc.h

Clean up the struct for hardware_path and drop the struct device_path
with a proper assignment of bc[] and mod members as signed chars.

This patch prepares for the kbuild change from Jason A. Donenfeld to
treat char as always unsigned.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>