tools: ynl: fix get_mask utility routine

Fix get_mask utility routine in order to take into account possible gaps
in the elements list.

Fixes: be5bea1c ("net: add basic C code generators for Netlink")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>