staging: erofs: fix if assignment style issue

Fix coding style issue "do not use assignment in if condition"
detected by checkpatch.pl.

Signed-off-by: Kristaps Čivkulis <kristaps.civkulis@gmail.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>