net: ipa: fix two mild warnings

Fix two spots where a variable "channel_id" is unnecessarily
redefined inside loops in "gsi.c".  This is warned about if
"W=2" is added to the build command.

Note that this problem is harmless, so there's no need to backport
it as a bugfix.

Remove a comment in gsi_init() about waking the system; the GSI
interrupt does not wake the system any more.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>