firmware: turris-mox-rwtm: Fix checking return value of wait_for_completion_timeout()

The wait_for_completion_timeout() function returns 0 if timed out, and a
positive value if completed. Fix the usage of this function.

Fixes: 389711b3 ("firmware: Add Turris Mox rWTM firmware driver")
Fixes: 2eab59cf ("firmware: turris-mox-rwtm: fail probing when firmware does not support hwrng")
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>