internal/ethapi: fix debug.chaindbProperty

[karalabe]

A while back we've switched ethdb.Database from a LevelDB backed struct to an interface. Some code was accidentally left in that still assumed the old structs/methods. This PR fixes on such instance where the debug.chaindbProperty got borked:

> debug.chaindbProperty("leveldb.stats")
WARN [07-18|16:08:30.756] Served debug_chaindbProperty             reqid=9 t=116.452µs err="chaindbProperty does not work for memory databases"
Error: chaindbProperty does not work for memory databases
    at web3.js:3143:20
    at web3.js:6347:15
    at web3.js:5081:36
    at <anonymous>:1:1

vs. this PR

> debug.chaindbProperty("leveldb.stats")
Compactions
 Level |   Tables   |    Size(MB)   |    Time(sec)  |    Read(MB)   |   Write(MB)
-------+------------+---------------+---------------+---------------+---------------
   0   |          2 |      53.29891 |       0.00000 |       0.00000 |       0.00000
   1   |         42 |      84.86589 |       0.00000 |       0.00000 |       0.00000
   2   |        567 |     954.72873 |       0.00000 |       0.00000 |       0.00000
   3   |       5112 |    9858.93917 |       0.00000 |       0.00000 |       0.00000
   4   |       2906 |    5625.94529 |       0.00000 |       0.00000 |       0.00000
-------+------------+---------------+---------------+---------------+---------------
 Total |       8629 |   16577.77798 |       0.00000 |       0.00000 |       0.00000