drivers: power: supply: goldfish_battery: Fix bogus SPDX identifier

spdxcheck.py complains:

 drivers/power/supply/goldfish_battery.c: 1:28 Invalid License ID: GPL

which is correct because GPL is not a valid identifier. Of course this
could have been caught by checkpatch.pl _before_ submitting or merging the
patch.

 WARNING: 'SPDX-License-Identifier: GPL' is not supported in LICENSES/...
 #19: FILE: drivers/power/supply/goldfish_battery.c:1:
 +// SPDX-License-Identifier: GPL

Which is absolutely hillarious as the commit introducing this wreckage says
in the changelog:

  There was a checkpatch complain:

    "Missing or malformed SPDX-License-Identifier tag".

Oh well. Replacing a checkpatch warning by a different checkpatch warning
is a really useful exercise.

Use the proper GPL-2.0 identifier which is what the boiler plate in the
file had originally.

Fixes: e75e3a12 ("drivers: power: supply: goldfish_battery: Put an SPDX tag")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/power/supply/goldfish_battery.c

-// SPDX-License-Identifier: GPL
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Power supply driver for the goldfish emulator
  *