wimax/i2400m: don't overwrite error codes when failing to load firmware

Make sure that i2400m_dev_bootstrap() doesn't overwrite the last known
error code with -ENOENT; when a firmware fails to load, we want to
know the cause and not a generic error code.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>