sign-file: fix the perl warning message when extracting ASN.1

There have the following warning message when running modules install
for sign ko files:

  # make modules_install
  ...
    INSTALL drivers/input/touchscreen/pcap_ts.ko
  Found = in conditional, should be == at scripts/sign-file line 164.
  Found = in conditional, should be == at scripts/sign-file line 161.
  Found = in conditional, should be == at scripts/sign-file line 159.

This patch change replace '=' by '==' in elsif conditions for avoid the
above warning messages.

Signed-off-by: Chun-Yi Lee <jlee@suse.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>