Bluetooth: BAP: Unicast client shall support 0 or > 1 endpoints

As per table 4.2 in BAP v1.0.1, the BAP unicast client shall
support at least 2 (> 1) endpoints in either direction if it
supports that direction.

Since Kconfig does not support such requirements (0 || > 1),
this is done using a BUILD_ASSERT.

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