parisc: pdc_stable.c: Cleaning up unnecessary use of memset in conjunction with strncpy

Using memset before strncpy just to ensure a trailing null character is
an unnecessary double writing of a string

Patch modified by Helge Deller to additionally reduce stack usage.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Helge Deller <deller@gmx.de>