usb: gadget: f_uac1: remove an unneeded NULL check

This NULL check sets off a static checker warning because we already
dereferenced "card" earlier in the function.  However, since "card" is
never NULL so we can just remove the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>