zd1201: remove unused variable framelen

Variable framelen is assigned but never read, hence it is redundant
and can be removed. Cleans up clang warning:

drivers/net/wireless/zydas/zd1201.c:234:3: warning: Value stored
to 'framelen' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>