Bluetooth: PACS: Fix support context if SRC/SNK disabled

If CONFIG_BT_PAC_SNK or CONFIG_BT_PAC_SRC was not enabled, the
function would not build due to missing Kconfig
(CONFIG_BT_PACS_SNK_CONTEXT or CONFIG_BT_PACS_SRC_CONTEXT).

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