uhd: fix timestamp conversion bug on 32-bit architectures
uhd: fix timestamp conversion bug on 32-bit architectures

Type size_t was used in the UHD time_spec_t to integer
conversion, which would overflow at roughly 4 and a half
hours causing the sample buffer to error on timestamp
validity. Builds where size_t takes on 64-bits were not
affected by this bug.

Signed-off-by: Thomas Tsou <ttsou@vt.edu>

git-svn-id: 504cad4147f1e500be0a169b9f46375eaf875953@2646 19bc5d8c-e614-43d4-8b26-e1612bc8e597

zabbal/openbts-p2.8,fairwaves/openbts-2.8,zabbal/openbts-p2.8,ttsou/openbts-p2.8,0x7678/openbts-2.8,fairwaves/openbts-2.8,0x7678/openbts-multi-arfcn,0x7678/openbts-2.8,zabbal/openbts-p2.8,0x7678/openbts-2.8,ttsou/openbts-p2.8,ttsou/openbts-p2.8,zabbal/openbts-p2.8,fairwaves/openbts-2.8,0x7678/openbts-multi-arfcn,zabbal/openbts-p2.8,0x7678/openbts-multi-arfcn,0x7678/openbts-multi-arfcn,kheimerl/vbts-openbts,0x7678/openbts-2.8,ttsou/openbts-p2.8,fairwaves/openbts-2.8,kheimerl/vbts-openbts,kheimerl/vbts-openbts,0x7678/openbts-multi-arfcn,kheimerl/vbts-openbts,ttsou/openbts-p2.8,kheimerl/vbts-openbts,fairwaves/openbts-2.8,0x7678/openbts-2.8