ath6kl: fix fw capability parsing

This patch fixes a bug where no capabilites are parsed when the number
of firmware capability bits translate into fewer bytes than the host has
knowledge of. Instead just process number of capability bytes as
reported by the firmware.

Signed-off-by: Thomas Pedersen <c_tpeder@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>