usb: gadget: Provide a default implementation of default manufacturer string

Some gadgets provide custom entry here. Some may override it with an
etntry that is also created by composite if there was no value sumbitted
at all.
This patch removes all "custom manufacturer" strings which are the same
as these which are created by composite. Then it moves the creation of
the default manufacturer string to usb_composite_overwrite_options() in
case no command line argument has been used and the entry is still an
empty string.
By doing this we get rid of the global variable "composite_manufacturer"
in composite.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>