staging: comedi: drivers: correct mode check for i8254_set_mode()

The upper bound check on the `mode` parameter of `i8254_set_mode()` and
`i8254_mm_set_mode()` is incorrect.  The `mode` parameter value consists
of a mode number in the range 0 to 5 in bits 3..1 {represented by the
constants `I8254_MODE0` (0 << 1) through to `I8254_MODE5` (2 << 1)} ORed
with a BCD/binary flag in bit 0 {represented by the constants
`I8254_BINARY` (0) and `I8254_BCD` (1)}.  The maximum allowed value
ought to be `I8254_MODE5 | I8254_BCD` ((5 << 1) | 1), but is currently
`I8254_MODE5 | I8254_BINARY` ((5 << 1) | 0).  Fix it.

None of the comedi drivers use `I8254_BCD` but some of the low-level
drivers allow user-space to configure the counter mode, so all legal
values ought to be allowed.  However, it's pretty unlikely anyone would
want to set the counters to count in BCD (binary coded decimal) so the
bug is not that significant.

Reported-by: Hartley Sweeten <HartleyS@visionengravers.com>
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>