Fix compability with libdivecomputer < 0.5.0

hw_ostc_device_fwupdate was introduced after 0.4.2 so ifdef it to
libdivecomputer >= 0.5.0. The scope of users interested in this feature
probably isn't big enough to justify a bump of our dependencies to a
unreleased version of libdivecomputer.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Dereference the right pointer in fw update slot

This fixes a copy-paste error, dereferencing the wrong pointer in the
slot that gets called when the firmware update thread has run.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Check return value after firmware updates

This reports back a error to the user if the firmware update failed.

We should re-wire this to have a log-function that gets called from
libdivecomputer via the context to get an actual error message.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>