diag: Check if usb is connected before usb_diag_write()

Check if usb is connected before writing to USB so
that there is no race condition  between usb_connected
variable update when usb is disconnected and
the time when we actually read the usb_connected variable.

Change-Id: I19135b0f7d1eb70e0aaa2be3beb62408aded34c2
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>