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

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

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

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

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>