ath6kl: Add debugfs entry to modify roaming parameters.

Firmware initiates roaming only after it reaches a rssi of 20.
This lower rssi threshold can be modified through a wmi command
to modify the roaming behavior.

kvalo: rename debugfs functions and move comment about rssi units next to
ath6kl_wmi_set_roam_lrssi_cmd()

Signed-off-by: Vivek Natarajan <nataraja@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>