core: fix minor bugs across arm_semihosting, EchoExample, mmio

[by965738071]

• arm_semihosting: fix FIle -> File typo in FType enum
• EchoExample: init tx_ready to true so first received packet can echo
• mmio: use @typename(FieldType) instead of @typename(T) in else branch so compile errors report the actual unsupported type, not 'void'

[mattnite]

Have you tested by running semihosting?

[Grazfather]

Semihosting is just a typo. I am more concerned about the USB change.

[by965738071]

Semihosting is just a typo. I am more concerned about the USB change.
这只是一个拼写错误的问题，没有必要进行测试验证。如果您有真正的技术顾虑，请提出来；否则，我们别在这上面浪费时间。