remoteproc/omap: remove the mbox_callback limitation

remoteproc/omap: remove the mbox_callback limitation

Now that remoteproc supports any number of virtio devices,
the previous sanity check in omap_rproc_mbox_callback
doesn't make sense anymore.

Remove that so we can start supporting multiple vdevs.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Brian Swetland <swetland@google.com>
Cc: Iliyan Malchev <malchev@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Mark Grosen <mgrosen@ti.com>
Cc: John Williams <john.williams@petalogix.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Loic PALLARDY <loic.pallardy@stericsson.com>
Cc: Ludovic BARRE <ludovic.barre@stericsson.com>
Cc: Omar Ramirez Luna <omar.luna@linaro.org>
Cc: Guzman Lugo Fernando <fernando.lugo@ti.com>
Cc: Anna Suman <s-anna@ti.com>
Cc: Clark Rob <rob@ti.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Saravana Kannan <skannan@codeaurora.org>
Cc: David Brown <davidb@codeaurora.org>
Cc: Kieran Bingham <kieranbingham@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>