usb: gadget: f_uac2: add short-hand for 'dev'

In afunc_bind() and afunc_set_alt(), &uac2->pdev.dev are used multiple
times. Adding a short-hand for them makes lines shorter so we can
remove some line wraps.

No functional change.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>