mwifiex: simplify length computation for some memset

This patch should be a no-op. It just simplifies code by using the name of
a variable instead of its type when calling 'sizeof'.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>