tplink-safeloader: use enum for table types

tplink-safeloader: use enum for table types

The partition table parser supports two table types, which only differ
in the starting ID of a table entry. Selection of which type to parse is
performed with a plain integer, but this makes code harder to read.
Replace the integer by an enum to make type selection more obvious.

While touching the code, also fix the switch-case indentation.

Signed-off-by: Sander Vanheule <sander@svanheule.net>