USB: serial: metro-usb: remove unneeded cast and function call

We should use kzalloc() instead of kmalloc() and memset(), and remove an
unneeded void * cast as well.

Cc: Aleksey Babahin <tamerlan311@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>