media: wl128x: get rid of a potential spectre issue

As reported by smatch:
	drivers/media/radio/wl128x/fmdrv_common.c:736 fm_irq_handle_rdsdata_getcmd_resp() warn: potential spectre issue 'rds_fmt.data.groupdatabuff.buff>

Address it by using array_index_nospec().

While here, reorder the linux/ includes.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>