mavlogdump.py: create local for m.get_type()

[peterbarker]

Before:

pbarker@fx:~/rc/logs$ time mavlogdump.py ./verloop/00000004.BIN -q 

real	0m34.787s
user	0m35.200s
sys	0m1.586s

pbarker@fx:~/rc/logs$ time mavlogdump.py ./verloop/00000004.BIN -q 

real	0m33.335s
user	0m33.683s
sys	0m1.664s
pbarker@fx:~/rc/logs$ 

pbarker@fx:~/rc/logs$ time mavlogdump.py ./verloop/00000004.BIN -q 

real	0m34.979s
user	0m35.332s
sys	0m1.642s
pbarker@fx:~/rc/logs$ 

After:

pbarker@fx:~/rc/logs$ time mavlogdump.py ./verloop/00000004.BIN -q 

real	0m32.996s
user	0m33.331s
sys	0m1.548s
pbarker@fx:~/rc/logs$ time mavlogdump.py ./verloop/00000004.BIN -q 

real	0m32.701s
user	0m33.073s
sys	0m1.639s
pbarker@fx:~/rc/logs$ 

So a few percent faster.

[khancyr]

LGTL
Pretty sure we already did the same on some other tools of mavproxy in the past, so that is fine