blobmsg: fix wrong payload len passed from blobmsg_check_array

blobmsg: fix wrong payload len passed from blobmsg_check_array

Fix incorrect use of blobmsg_len() on passed blobmsg to
blobmsg_check_array_len() introduced in commit 379cd33d1992
("fix wrong payload len passed from blobmsg_check_array") by using correct
blob_len().

By using blobmsg_len() a value too small was passed to blobmsg_check_array()
which could lead to this function returning an error when there is none.

Fixes: 379cd33d1992 ("fix wrong payload len passed from blobmsg_check_array")
Signed-off-by: Chris Nisbet <nischris@gmail.com>
[add fixes tag, rewrap commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>