usb: gadget: ncm: Fix string data index definition

Commit 489895b1 renamed the
local definition of STRING_DATA_IDX to NCM_STRING_DATA_IDX
to avoid a namespace conflict, but the corresponding usage
sites have not been updated.

Change-Id: Ic1b6671d3687a21f399c26b557a02a55a8e27cfe
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>