Bluetooth: CAP: Fix issue with parallel CAP discover

The bt_cap_common_discover function relied on a global variable
used to  indicate that a discovery was in process.
This global variable prevented multiple discoveries to take place
on multiple ACL connections, where the intention was to stop
multiple discoveries on the same ACL.
This has been fixed by moving the variable into the
struct bt_cap_common_client, so that it applies per
connection, rather than a global check.

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