staging: silicom: fix sparse warning for static variable

This patch fixes the following sparse warning in bpctl_mod.c:
warning: symbol 'bpvm_lock' was not declared. Should it be static?

Signed-off-by: James A Shackleford <shack@linux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>