(lwla_setup_acquisition): Set the clock divider bypass
flag to 1 for the external clock modes as well.
(capture_setup): Set the clock divide count to 0 if an
external clock source is selected.
(lwla_set_clock_source): Checking whether an enum value is greater
than or equal to zero apparently results in a warning with some
compilers. Assign the enum to an unsigned variable to avoid this,
and return SR_ERR_BUG if the range is exceeded, as this indicates
a bug in the driver code itself.
Daniel Elstner