staging: comedi: amplc_pc236: Simplify PCI board look-up

Since only a single PCI board is supported by the driver, there is no
need to call `pc236_find_pci_board()` to find the a board entry with
matching PCI device ID in `pc236_pci_boards[]`.  Just point to the entry
directly and remove the look-up function.  In fact, there is no reason
for `pc236_pci_boards[]` to be an array, so change it to a non-array
variable and rename it to `pc236_pci_board`.  Also, the `devid` member
of `struct pc236_board` is no longer needed as it was only used by the
look-up function, so remove it.

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