part_efi: make sure the gpt_pte is freed

the gpt_pte wasn't being freed if it was checked against an invalid
partition. The resulting memory leakage could make it impossible
to repeatedly attempt to load non-existent files in a script.

Also, downgrade the message for not finding an invalid partition
from a printf() to a debug() so as to minimize message spam in
perfectly normal situations.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>