ath6kl: Make use of return value from ath6kl_diag_read()

In ath6kl_read_fwlogs(), return value from ath6kl_diag_read()is not
used to bail out in case of any errors in reading fw log. No real issue
is observed because of this, reported by source code analyzer.

kvalo: fix a long line warning

Signed-off-by: Pandiyarajan Pitchaimuthu <c_ppitch@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>