libbladeRF: [WIP] Changed API interfaces to use int instead of ssize_t.

This is intended to simplify some portability issues, since an int
type is sufficient in the few spots where we had ssize_t.

We'll need to still add a ssize_t wrapper around the Windows
[SSIZE_T](msdn.microsoft.com/en-us/library/windows/desktop/aa383751(v=vs.85).aspx#SSIZE_T)
for use inside the library.