usb: gadget: f_uac2: Neaten and reduce size of afunc_validate_opts

usb: gadget: f_uac2: Neaten and reduce size of afunc_validate_opts

Remove the repetition and reduce the object size a bit.

$ size drivers/usb/gadget/function/f_uac2.o* (x86-64 defconfig with gadget)
   text	   data	    bss	    dec	    hex	filename
  24515	   3136	     16	  27667	   6c13	drivers/usb/gadget/function/f_uac2.o.new
  24817	   3136	     16	  27969	   6d41	drivers/usb/gadget/function/f_uac2.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Link: https://lore.kernel.org/r/2132d97ca8d4dd5ac9426cc23af95e819079b02c.camel@perches.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>