Staging: silicom: fix dangerous if condition in bpctl_mod.c

checkpatch script returns the following warning:
ERROR: do not use assignment in if condition
+                       if (((dev_num = get_dev_idx(dev->ifindex)) ==
            -1) |

This patch fixes the if condition by splitting it.

Signed-off-by: Neil Armstrong <superna9999@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>