paride: fix up build warning on mips platforms

MIPS include files define "PC" so when building the paride driver the
following build warning shows up:

	rivers/block/paride/bpck.c:32: warning: "PC" redefined

Fix this by undefining PC before redefining it as is done for other
defines in this driver.

Cc: Tim Waugh <tim@cyberelk.net>
Acked-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20211130084626.3215987-1-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/block/paride/bpck.c

@@ -28,6 +28,7 @@
 
 #undef r2
 #undef w2
+#undef PC
 
 #define PC			pi->private
 #define r2()			(PC=(in_p(2) & 0xff))