usb: gadget: function: fix dangling pnp_string in f_printer.c

When opts->pnp_string is changed with configfs, new memory is allocated for
the string. It does not, however, update dev->pnp_string, even though the
memory is freed. When rquesting the string, the host then gets old or
corrupted data rather than the new string. The ieee 1284 id string should
be allowed to change while the device is connected.

The bug was introduced in commit fdc01cc2 ("usb: gadget: printer:
Remove pnp_string static buffer"), which changed opts->pnp_string from a
char[] to a char*.
This patch changes dev->pnp_string from a char* to a char** pointing to
opts->pnp_string.

Fixes: fdc01cc2

 ("usb: gadget: printer: Remove pnp_string static buffer")
Signed-off-by: Albert Briscoe <albertsbriscoe@gmail.com>
Link: https://lore.kernel.org/r/20220911223753.20417-1-albertsbriscoe@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>