mailbox: Remove const from client argument of mbox_request_channel()

mailbox: Remove const from client argument of mbox_request_channel()

The struct mbox_client supplied to mbox_request_channel() is const but
it is stored in the channel in a non-constant member causing compiler
warnings.

While the mailbox API should treat the struct mailbox_client as const
itself the struct is passed back to the channel in callbacks without
a const so we need to either remove the const, change the callbacks to
take const or cast the const away when doing callbacks. Take the simplest
option and just remove the const.

Signed-off-by: Mark Brown <broonie@linaro.org>