wcn36xx: Increase number of TX retries

Increase the short/long retry limit to 15 in order to impove TX
robustness in noisy/busy environment. 15 is the default value
defined in the downstream driver. Observed number of ack timeout
is reduced with this change.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1595586052-16081-4-git-send-email-loic.poulain@linaro.org