Bluetooth: ISO: Update loop variables types in iso.c
Bluetooth: ISO: Update loop variables types in iso.c

Some of the loop variables used `int` where it
makes more sense to use `size_t` or `uint8_t`.

Signed-off-by: Emil Gydesen <a20314356615223738233ab4c4eb1cd8cd4146fa@nordicsemi.no>

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