comedi: drivers: ni_routes: Use strcmp() instead of memcmp()

The family and device comparisons were using memcmp(), but this could
lead to Out-of-bounds reads when the length was larger than the
buffers being compared. Since these appear to always be NUL-terminated
strings, just use strcmp() instead.

This was found with Clang under LTO:

[ 92.405851][    T1] kernel BUG at lib/string_helpers.c:980!
...
[ 92.409141][ T1] RIP: 0010:fortify_panic (fbdev.c:?)
...
[ 92.410056][ T1] ni_assign_device_routes (fbdev.c:?)
[ 92.410056][ T1] ? unittest_enter (fbdev.c:?)
[ 92.410056][ T1] ni_routes_unittest (ni_routes_test.c:?)
[ 92.410056][ T1] ? unittest_enter (fbdev.c:?)
[ 92.410056][ T1] __initstub__kmod_ni_routes_test__505_604_ni_routes_unittest6 (fbdev.c:?)
[ 92.410056][ T1] do_one_initcall (fbdev.c:?)

Link: https://lore.kernel.org/lkml/20220210072821.GD4074@xsang-OptiPlex-9020
Fixes: 4bb90c87 ("staging: comedi: add interface to ni routing table information")
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley...