mwifiex: fix the length parameter of a memset

In 'mwifiex_get_ver_ext', we have:
   struct mwifiex_ver_ext ver_ext;

   memset(&ver_ext, 0, sizeof(struct host_cmd_ds_version_ext));

This is likely that memset'ing sizeof(struct mwifiex_ver_ext) was expected.
Remove the ambiguity by using the variable name directly instead of its
type.

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