leaudio: Make LE Connection Interval a multiple of SDU_Interval

Updating CI to 60 msec which is in multiple of both (7.5 and 10 msec)
SDU_Interval. This will help improve scheduling of CIS and associated LE
connection.

Test: atest --host bluetooth_le_audio_client_test

Change-Id: I49e9719b3359a182a88d1c66145408091fbe4d36