usb: frmnet: fix the NULL pointer access crash by using global rmnet dev

Transport_store function is trying to access the rmnet usb_function
from struct device pointer and ending up in retreiving the false
rmnet dev pointer as struct device pointer won't be having
rmnet usb_function.
Fix this by using the global rmnet dev pointer.

Change-Id: I8be5ba3ca4315a48ed956a29186f54ae6d6c2674
Signed-off-by: Anji jonnala <anjir@codeaurora.org>