USB: serial: mos7720: clean up urb->status usage

This done in anticipation of removal of urb->status, which will make
that patch easier to review and apply in the future.


Cc: <linux-usb-devel@lists.sourceforge.net>
Cc: VijayaKumar G.N. <vijaykumar@aspirecom.net>
Cc: AjayKumar <ajay@aspirecom.net>
Cc: Gurudeva N. <gurudev@aspirecom.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>