USB: replace hardcode maximum usb string length by definition

Replace hardcoded maximum USB string length (126 bytes) by definition
"USB_MAX_STRING_LEN".

Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/1592471618-29428-1-git-send-email-macpaul.lin@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>