remoteproc: fix error return code in rproc_fw_boot()

Set 'ret' to -EINVAL when needed, so a sensible return value
is returned on errors.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
[fix additional instances of this bug as well, update commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>