serial: max3100: Replace open-coded parity calculation with parity8()

Refactor parity calculations to use the standard parity8() helper.
This change eliminates redundant implementations.

Co-developed-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Link: https://lore.kernel.org/r/20250515081311.775559-1-visitorckw@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: visitorckw

drivers/tty/serial/max3100.c

@@ -16,6 +16,7 @@
 /* 4 MAX3100s should be enough for everyone */
 #define MAX_MAX3100 4
 
+#include <linux/bitops.h>
 #include <linux/container_of.h>
 #include <linux/delay.h>
 #include <linux/device.h>
@@ -133,7 +134,7 @@ static int max3100_do_parity(struct max3100_port *s, u16 c)
 	else
 		c &= 0xff;
 
-	parity = parity ^ (hweight8(c) & 1);
+	parity = parity ^ parity8(c);
 	return parity;
 }