fix(mhuv2): provide only the usable size of memory

The function mhu_get_max_message_size() for MHUv2 should return only the
available memory for use after considering all the overheads for its own
use.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I14ad16e8f4b781e396bca6173077513db74157d5

drivers/arm/mhu/mhu_wrapper_v2_x.c

@@ -308,5 +308,10 @@ size_t mhu_get_max_message_size(void)
 
 	assert(num_channels != 0);
 
-	return num_channels * sizeof(uint32_t);
+	/*
+	 * Returns only usable size of memory. As one channel is specifically
+	 * used to inform about the size of payload, discard it from avialable
+	 * memory size.
+	 */
+	return (num_channels - 1) * sizeof(uint32_t);
 }