rpcd-mod-luci: fix reporting network device flags

Fix reporting of ceertain flag values larger than 255, such as IFF_PROMISC
by explicitly casting the bit test expression to a boolean result since
the implicit integer truncation to uint8_t will turn the `0x100` result of
a set IFF_PROMISC bit into just `0x0`.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>