usb: Guarantee alignment of the string descriptor

usb: Guarantee alignment of the string descriptor

GCC 9.x starts complaining about potential misalignment of the pointer to the
array (in this case alignment=2) in the packed (alignment=1) structures.

drivers/usb/gadget/composite.c:545:23: warning: taking address of packed member of ‘struct usb_string_descriptor’ may result in an unaligned pointer value [-Waddress-of-packed-member]
  545 |    collect_langs(sp, s->wData);

drivers/usb/gadget/composite.c:550:24: warning: taking address of packed member of ‘struct usb_string_descriptor’ may result in an unaligned pointer value [-Waddress-of-packed-member]
  550 |     collect_langs(sp, s->wData);

drivers/usb/gadget/composite.c:555:25: warning: taking address of packed member of ‘struct usb_string_descriptor’ may result in an unaligned pointer value [-Waddress-of-packed-member]
  555 |      collect_langs(sp, s->wData);

Define structure alignment to be 2 to guarantee alignment of its members.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[sw0312.kim: apply from list https://patchwork.ozlabs.org/patch/1155376/, to remove gcc9 warning]
Suggested-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: Id5b0f65266f452458623c49c46dee002c462a3b2