staging: comedi: addi_apci_3501: simplify reading the eeprom

The only value in the eeprom that is used by this driver is the
number of analog output channels.

Copy the necessary code from addi_eeprom.c to this driver and
refactor it so that we can get the value needed.

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>