brcmfmac: of: remove redundant variable len

The variable len is being assigned bit is never used. The variable
and the strlen call are redundant and can be removed.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220207133329.336664-1-colin.i.king@gmail.com