Suppress MISRA 14.3 finding in ARP buffer allocation

Move the buffer size ternary into a named macro
(arpOUTPUT_REQUEST_BUFFER_SIZE) with a coverity suppression comment.
The expression compares sizeof against a configuration constant and is
invariant for a given build but varies across user configurations.

Author: patrzhan

MISRA.md

@@ -199,6 +199,9 @@ _Ref 14.3.1_
 
 - MISRA C-2012 Rule 14.3 False positive as the value might be changed depending
   on the conditionally compiled code
+- MISRA C-2012 Rule 14.3 The controlling expression compares sizeof against a
+  configuration constant. The result is invariant for a given configuration but
+  varies across different user configurations.
 
 #### Rule 17.2
 

source/FreeRTOS_ARP.c

@@ -1194,6 +1194,9 @@
         {
             /* This is called from the context of the IP event task, so a block time
              * must not be used. */
+            /* MISRA Ref 14.3.1 [Invariant controlling expression] */
+            /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-143 */
+            /* coverity[misra_c_2012_rule_14_3_violation] */
             pxNetworkBuffer = pxGetNetworkBufferWithDescriptor( ( ( sizeof( ARPPacket_t ) > ( size_t ) ipconfigETHERNET_MINIMUM_PACKET_BYTES ) ? sizeof( ARPPacket_t ) : ( size_t ) ipconfigETHERNET_MINIMUM_PACKET_BYTES ), ( TickType_t ) 0U );
 
             if( pxNetworkBuffer != NULL )