[CANN EP] Fix Float16 support for CANN EP (#16733)
### Description
<!-- Describe your changes. -->
Replace the constructor function `MLFloat16()` with the public member
function `FromBits()` in the file
`onnxruntime/core/providers/cann/cann_common.cc`
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
PR [#16506](https://github.com/microsoft/onnxruntime/pull/16506) changed
the public constructor function `MLFloat16(uint16_t x)` to private, and
added a public function `MLFloat16::FromBits(uint16_t x)` in the file
`include/onnxruntime/core/framework/float16.h`, which broke the CANN CI.
This PR aligns the CANN behavior with the modified class `MLFloat16`.
