net: wwan: iosm: fixes unable to send AT command during mbim tx

ev_cdev_write_pending flag is preventing a TX message post for
AT port while MBIM transfer is ongoing.

Removed the unnecessary check around control port TX transfer.

Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>