fs/squashfs: sqfs_probe: use sqfs_decompressor_init() return value

fs/squashfs: sqfs_probe: use sqfs_decompressor_init() return value

sqfs_decompressor_init() returns a value, so it's better to use it than
to force the return value to EINVAL (it could be ENOMEM)

Reviewed-by: Joao Marcos Costa <jmcosta944@gmail.com>
Signed-off-by: Richard Genoud <richard.genoud@posteo.net>