wifi: iwlegacy: remove redundant variable len

Variable len is being assigned and modified but it is never
used. The variable is redundant and can be removed.

Cleans up clang scan build warning:
warning: variable 'len' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221104135036.225628-1-colin.i.king@gmail.com