Fix ti CCS compiler errors

Description

Fixed some ti compiler related issues:

• source\FreeRTOS_DNS_Parser.c, line 278:
  error 551-D: variable "xDNSHookReturn" is used before its value is set
  --> 265: BaseType_t xDNSHookReturn = 0U;

• source\FreeRTOS_IPv6.c, line 102:
  error 193-D: type qualifier is meaningless on cast type
  --> 102: const uint8_t * const pucEthernetBuffer = ( const uint8_t *) pvEthernetBuffer;

• same in:

  • source\FreeRTOS_IPv6.c, line 106
  • source/FreeRTOS_IPv4.c, line 85
  • source\FreeRTOS_RA.c, line 332:

• error 552-D: variable "ulMTU" was set but never used
  --> guard by ifdef ipconfigHAS_PRINTF

• removed vNDSendUnsolicited() as described in https://forums.freertos.org/t/unused-variable-in-freertos-nd-c-xlastunsolicitedndtime/23347

Test Steps

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

#1273

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