parisc: fix preempt_count() check in entry.S

preempt_count in struct thread_info is unsigned int,
but the entry.S code used LDREG, which generates a 64 bit
load when compiled for 64 bit. Fix this to use an ldw and
also change the condition in the compare one line below
to only compares 32 bits, although ldw zero extends, and
that should work with a 64 bit compare.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>