homework: cleanup use of ERRNO_IS_DEVICE_ABSENT()

homework: cleanup use of ERRNO_IS_DEVICE_ABSENT()

Given that ERRNO_IS_DEVICE_ABSENT() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the arguments passed to ERRNO_IS_DEVICE_ABSENT()
are the values returned by external cryptsetup functions
sym_crypt_init_by_name() and sym_crypt_deactivate_by_name() which are
not expected to return any positive values, but let's be consistent
anyway and move ERRNO_IS_DEVICE_ABSENT() invocations to the branches
where the return values are known to be negative.