core: Switch usbi_transfer to store timeout as timespec

core: Switch usbi_transfer to store timeout as timespec

The transfer timeout is structured around time values provided by the
clock_gettime() function. This function uses a timespec structure, but
the usbi_transfer structure was storing its calculated timeout in a
timeval structure. This mismatch introduces extra work when checking for
transfer timeouts as there must be a conversion between these two
structures. Eliminate this by storing the calculated timeout as a
timespec, thus allowing direct comparison.

Note that a conversion to a timeval is still necessary in the
libusb_get_next_timeout() function because the public API uses a timeval
structure, but this is now the only place where such a conversion is
done.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>