use the same data types as in i2c_phdr

Change-Id: Ic0abcf8173a690a1dc0cd250f5e8770eb92a5aa9
Reviewed-on: https://code.wireshark.org/review/5626
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>