Bluetooth: PACS: Set PACS read_buf size to BT_ATT_MAX_ATTRIBUTE_LEN

The maximum size of the characteristic is BT_ATT_MAX_ATTRIBUTE_LEN.
The previous size limited the size of the characteristic to
the maximum possible MTU, thus not utlizing support for the
Read Long procedure.

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