cosmetic: uhd: Move smpl_buf out of UHDDevice, move UHDDevice class definition to .h

* move class definition to .h file, like we do for other devices.
* move smpl_buf class to a different file inside uhd/.
* Preparation work to have smpl_buf being used in a generic way for
devices other than UHD (LMS).

Change-Id: Ib4594320da9bb7f6e9f52e7d70d11ecd11106aae
5 files changed