staging: comedi: daqboard2000: check firmware length

Firmware files for DAQBoard/2000 have a header, which is skipped,
followed by a sequence of FPGA configuration bytes to be programmed in
pairs.  The FPGA configuration bytes start with the sequence 0xff, 0x20.

Make the firmware loading callback function
`daqboard2000_load_firmware()` return an error `-EINVAL` if the FPGA
start sequence is not found, or the remaining length is not a multiple
of 2.

The firmware loading callback tries to program the FPGA up to 3 times
until it succeeds or it has tried too many times.  Currently, it
searches for the FPGA start sequence in the firmware data each time
through the retry loop.  Change it to adjust the start position and
length before entering the loop.

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>