USB: serial: usb_wwan: remove redundant assignment to variable i

Variable i is being assigned a value that is never read, it is being
re-assigned two statements later in a for-loop. The assignment is
redundant and can be removed.

Cleans up clang scan build warning:
drivers/usb/serial/usb_wwan.c:151:2: warning: Value stored to 'i'
is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>