staging: comedi: rtd520: remove unnecessary interrupt disable

The read/write of the PLX_INTRCS_REG during the driver detach is not
necessary. The rtd_reset() function writes 0 to this register which
will disable all interrupts.

This also fixes a dereference after null check reported by coverity.

Reported-by: coverity (CID 751066)
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>