[UWB HAL] Use AsyncFd to read the buffer only when it's readable.

This CL fixes the issue that some bytes are missing after we toggle uwb
off and on.

Test: atest CtsUwbTestCases
Bug: 302005209

Change-Id: I8ad8072fe01c8d1f466d8debf0ccf5831b3eddf8
2 files changed