Bluetooth: CAP: Fix linker issue in cap_stream.c for x86

The existing code in the cap_stream.c that handled the
check before calling CAP initiator unicast functions
seemingly did not work for x86 targets such as native_sim
or native_posix.

Modified the check so that IS_ENABLED is used directly.

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