staging: comedi: ni_tio: remove BUG() checks for ni_tio_get_clock_src()

This function calls some helper functions to convert the counter variant
specific clock select bits into the generic enum ni_gpct_clock_source_bits
equivelent. These helper functions currently BUG() if the clock select
bits are invalid.

It then calls ni_tio_clock_period_ps() to figure out the clock period
based on the generic clock source. This function could also BUG() if
the prescale bits are invalid.

In reality this should never happen but refactor the code to return
-EINVAL instead and remove the BUG() checks.

These functions are also called by ni_tio_set_sync_mode(). When this
function is called by ni_tio_set_clock_src() the counter select bits
have already been validated.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>