slimbus: make functions slim_ack_txn and slim_alloc_txbuf static

The functions slim_ack_txn and slim_alloc_txbuf are local to the
source and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
symbol 'slim_ack_txn' was not declared. Should it be static?
symbol 'slim_alloc_txbuf' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>