net: sockets: do not unconstify 'optval' in setsockopt()

'optval' in setsockopt(..., SO_BINDTODEVICE, ...) was casted explicitly
from 'const void *' to 'struct ifreq *'. Rely on C implicit casting from
'const void *' to 'const struct ifreq *' and simply update variable
type. This prevents unwanted modification of ifreq value in the future.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>