Bluetooth: BAP: Fix BASE_SUBGROUP_MAX_COUNT value

The calculation for BASE_SUBGROUP_MAX_COUNT was off by one.
The macro did not consider the non-subgroup parameters of
the BASE that take up space. This effectively modifies
BASE_SUBGROUP_MAX_COUNT to be 24 instead of 25.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>