usb: device_next: Fix string descriptors response

Commit 2f31ee63 ("usb: device_next: convert ASCII7 strings to
UTF16LE on the fly") made string descriptors respond with twice as much
of the actual string data.

Fix the issue by taking into account that USB string descriptor length
is already multiplied by two. Additionally, make it possible to return
odd number of bytes if host requested so.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>