staging: comedi: usbduxfast: rename CamelCase 'minSamplPer'

Rename the local variable to fix the checkpatch.pl warning.

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