ath6kl: refactor firmware load address code

Currently the load address was calculated everytime when it was needed,
and with a mess if clauses. Simplify this by adding a field to struct
ath6kl for each address and choose the address with simple switch
statements.

Also move the code just after target version is retrieved. That way it's
easier to override the values later in the boot process.

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