Bluetooth: ISO: Fix issue with ++ in calls to sys_le16_to_cpu

sys_le16_to_cpu is a macro, and not big endian the value
is referenced twice, which would incorrectly increment the
value.

Modifed to extract the value to a variable before calling
sys_le16_to_cpu.

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