Fix MISRA Rule 5.7 Part 2

Follow convention of shorter names for smaller scope to fix
violations of MISRA rule 5.7

To prevent violation of directive 4.5 having variable name channel
in css_pm_scmi.c not being typographically ambiguous change macro
argument CHANNEL in css_mhu_doorbell.h change argument to _channel
to fit with our convention which is a permitted exception of
directive 4.5 for this project

Rule 5.7: A tag name shall be a unique identifier

Fixed for:
    make LOG_LEVEL=50 PLAT=juno

Change-Id: I147cdb13553e83ed7df19149b282706db115d612
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
diff --git a/plat/arm/css/drivers/mhu/css_mhu_doorbell.h b/plat/arm/css/drivers/mhu/css_mhu_doorbell.h
index 3c94536..cb75ed0 100644
--- a/plat/arm/css/drivers/mhu/css_mhu_doorbell.h
+++ b/plat/arm/css/drivers/mhu/css_mhu_doorbell.h
@@ -18,8 +18,8 @@
 #define MHU_V2_ACCESS_REQ_OFFSET		0xF88
 #define MHU_V2_ACCESS_READY_OFFSET		0xF8C
 
-#define SENDER_REG_STAT(CHANNEL)	(0x20 * (CHANNEL))
-#define SENDER_REG_SET(CHANNEL)		(0x20 * (CHANNEL)) + 0xC
+#define SENDER_REG_STAT(_channel)	(0x20 * (_channel))
+#define SENDER_REG_SET(_channel)	((0x20 * (_channel)) + 0xC)
 
 /* Helper macro to ring doorbell */
 #define MHU_RING_DOORBELL(addr, modify_mask, preserve_mask)	do {	\