staging: comedi: usbduxfast: remove unused variable in usbduxfastsub_ai_Irq()

The variable 'p' is initialized but never used otherwise, so
remove the unused variable.

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>