iio: scmi: fix spelling of SPDX tag

checkpatch looks for SPDX-License-Identifier.
Remove the extra spaces.
Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20210610214438.3161140-7-trix@redhat.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/common/scmi_sensors/Makefile

-# SPDX - License - Identifier : GPL - 2.0 - only
+# SPDX-License-Identifier: GPL-2.0-only
 #
 # Makefile for the IIO over SCMI
 #