staging: comedi: s626: remove unnecessary variable initialization

We initialize 'irqbit' to 0, only to properly set it immediately
afterwards.  Just remove the zero-initialization.

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