staging: comedi: addi_eeprom: move the eeprom offset to addi_eeprom_readw()

All the addi_eeprom_readw() calls have the offset to the start of user
data (0x100) as part of the address calculation. Create a define for this
constant and move it's addition to the address into addi_eeprom_readw().

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>