cmd_bootm.c: Correct check/return for unsupported sub-command

With the do_bootm_states re-organization, we have the call to any
potential sub-commands in a single spot.  If one fails, we can then stop
right there and return to the caller.  Prior to these calls we have
already ensured that ret is zero so we will not be returning this error
for some other case.

Signed-off-by: Tom Rini <trini@ti.com>
Tested-by: Andreas Bießmann <andreas.devel@googlemail.com>