dev: Integrate `promisc' module into `dev' module

Since entering/leaving promiscuous mode also is a device specific
function and all users of the `promisc' module also use `dev', integrate
it there. Also rename the functions to have a `device_' prefix like the
other functions in the module.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
pcap_rw: Fix compiler warnings

Fix the following compiler warnings that occur when building with "-W
-Wall -Wextra":

pcap_rw.c: In function ‘pcap_rw_write’:
pcap_rw.c:30:6: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
pcap_rw.c: In function ‘pcap_rw_read’:
pcap_rw.c:50:6: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>