Staging: rtl8192e: Replace license text with SPDX identifier

- adds the SPDX GPL-2.0 license identifier which solves the checkpatch.pl warning
- Removed license boilerplate text
- Issue found with checkpatch.pl warning:
  "WARNING: Missing or malformed SPDX-License-Identifier tag in line 1"

Signed-off-by: Bhanusree Pola <bhanusreemahesh@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>