convert callback

This patch allow us to efficiently modify the number of
remaining conversions from the client side. This us useful
when we do not know in advance how many conversions we will
need or when we need to cancel pending conversions.

This change is simple enough to be compatible with existing
code that can just define the new pointer in the callback
and ignore it.

Sample usage:

http://tinyurl.com/s3c2410-ts-c (function stylus_adc_action).
Signed-off-by: Nelson Castillo <arhuaco@freaks-unidos.net>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>