ath10k: use the right length of "background"

The word "background" contains 10 characters so the third argument of
strncmp() need to be 10 in order to match this prefix correctly.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Fixes: 855aed12 ("ath10k: add spectral scan feature")
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>