wifi: ath11k: use kstrtoul_from_user() where appropriate

Use 'kstrtoul_from_user()' in 'ath11k_write_file_spectral_count()'
and 'ath11k_write_file_spectral_bins()'

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230824075121.121144-4-dmantipov@yandex.ru