spki: fix checkpatch coding style warnings
spki: fix checkpatch warning: else is not generally useful after a return

When return is used in the if branch, the else branch can be removed.
spki: remove unnecessary cast of void pointer in key_entry_cmp()

When a void * is assigned to pointer of another type, it's not required
to do a cast. It also fixes that the const type of the parameters were
removed in the cast.
build: remove Cflags options from tests/CMakeLists.txt

In commit 579b0ced6104a5b5cb0ba8f6d7220b568c1db3b6, Cflags were added to
tests/CMakeLists.txt. This overwrites the Cflag that are set from the
top-level makefile.
passing "-D Release" or "-D Debug" to cmake didn't worked anymore as
expected, because the cflags were overwritten.
spki: change return value to void of function key_entry_to_spki_record()

The function always returned success and no error checking happened in
the function. Therefore change the return value to void.
spki: change return value to void of function spki_record_to_key_entry()

The function always returned success and no error checking happened in
the function. Therefore change the return value to void.
spki: use sizeof() on variable that is allocated
spki: use out label to exit function also in successful case

spki_table_remove_entry() had a goto label defined to unlock the lock
and leave the function. Use this label also in the successful case,
instead of doing unlock and returning in that specific if-branch.
spki: remove unnecessary cast of void PTR