ath6kl: initialize the 'nominal_phy' field in the 'wmi_create_pstream_cmd' struct for create_qos command

The nominal_phy field is uninitialized. Initialize it to min_phy_rate for
create_qos.

kvalo: simplified the equation as checkpatch complained for a too long line
Signed-off-by: Chilam Ng <chilamng@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>