remoteproc/omap: revise a minor error trace message

The omap_mbox_msg_send() is the legacy API for sending a mailbox
message. It has been replaced with the mbox_send_message() from
the mailbox framework. Revise the failure trace to print a generic
failure message instead of referencing the actual function name.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>