staging: comedi: Check length of INSN_CONFIG_TIMER_1 instruction

The contents of the Comedi configuration instruction
`INSN_CONFIG_TIMER_1` instruction are not very well defined, but the one
driver that uses it (the "cb_pcidas64" driver for the PCI-DAS4020/12
card) assumes its `insn->n` is 5. Add a check in
`check_insn_config_length()` to verify that `insn->n` is correct for
this configuration instruction.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>