Make Flags of proper width

[AniruddhaKanhere]

The TCP flags are not of proper width according to the header in the TCP protocol.

Description

The TCP flags are not of proper width according to the header in the TCP protocol. In absence of a cast, this value is downcasted from uin16_t to uint8_t to assign value to the 8 bit width xTCP_HEADER.ucTCPFlags.
These flags are designed to fit in a 6-bit wide value according to RFC 793. The other bits are experimental and are defined in various RFCs. They are not used as of now in the +TCP stack except for the purposes of debugging.

The issue that triggered this PR:
Visual Studio build (v142) yields 2 warnings for FreeRTOS_TCP_IP.c file, both of which are cast truncates constant value.

1>amaon-freertos\amazon-freertos\libraries\freertos_plus\standard\freertos_plus_tcp\source\FreeRTOS_TCP_IP.c(864,60): warning C4310: cast truncates constant value
1>amazon-freertos\libraries\freertos_plus\standard\freertos_plus_tcp\source\FreeRTOS_TCP_IP.c(1885,56): warning C4310: cast truncates constant value

Both places downcast macro constants from uin16_t to uint8_t to assign value to the 8-bit width xTCP_HEADER.ucTCPFlags member here

Checklist:

• I have tested my changes. No regression in existing tests.
• My code is Linted.

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