copy-firmware: Fix test: unexpected operator

dash is the default /bin/sh on Debian based system.

```
dash ./copy-firmware.sh --zstd example
./copy-firmware.sh: 37: test: unexpected operator
```

`=` should be used with the test command for POSIX conformance.

Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>

copy-firmware.sh

@@ -24,7 +24,7 @@ while test $# -gt 0; do
             ;;
 
         --xz)
-            if test "$compext" == ".zst"; then
+            if test "$compext" = ".zst"; then
                 echo "ERROR: cannot mix XZ and ZSTD compression"
                 exit 1
             fi
@@ -34,7 +34,7 @@ while test $# -gt 0; do
             ;;
 
         --zstd)
-            if test "$compext" == ".xz"; then
+            if test "$compext" = ".xz"; then
                 echo "ERROR: cannot mix XZ and ZSTD compression"
                 exit 1
             fi