dmaengine: idxd: correct reserved token calculation

The calcuation for limit of reserved token did not take into account the
change the user wanted vs the current group reserved token. This causes
changing of the reserved token to be possible only after we set the value
of the reserved token back to 0. Fix calculation so we can set a value that
is non zero for reserved token.

Fixes: c52ca478

 ("dmaengine: idxd: add configuration component of driver")
Reported-by: Jerry Chen <jerry.t.chen@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/158204471889.37789.7749177228265869168.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>