Bluetooth: BAP: Fix CONFIG_BT_BAP_BASS_MAX_SUBGROUPS include issue

There were a few cases where CONFIG_BT_BAP_BASS_MAX_SUBGROUPS was
used in structs in the public header files. Unfortunately using
a Kconfig for an array size will cause include issues if the
file is included without the Kconfig being available.

For now add and use a new #define value that will be 0 if
CONFIG_BT_BAP_BASS_MAX_SUBGROUPS is not defined.

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