staging: comedi: hwrdv_apci3501: absorb private header

The header file hwdrv_apci3501.h is only included by the source
file hwrdv_apci3501.c. Absorb the header into the source and
delete the header.

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