driver core: platform: need consistent spacing around '-'

Fix the below checkpatch issue:

ERROR: need consistent spacing around '-' (ctx:WxV)
FILE: drivers/base/platform.c:1008:
+       len = acpi_device_modalias(dev, buf, PAGE_SIZE -1);
                                                       ^

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Link: https://lore.kernel.org/r/20200602045556.66948-1-song.bao.hua@hisilicon.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>