usb: gadget: u_audio: Fix the size of a buffer in a strscpy() call

usb: gadget: u_audio: Fix the size of a buffer in a strscpy() call

The size given to strscpy() is not consistent with the destination buffer
that is used. The size is related to 'driver' and the buffer is
'mixername'.

sizeof(card->mixername) is 80 and sizeof(card->driver) is 16, so in
theory this could lead to unneeded string truncation.

In practice, this is not the case because g_audio_setup() has only 2
callers. 'card_name' is either "UAC1_Gadget" or "UAC2_Gadget".

Anyway, using the correct size is cleaner and more future proof.

In order to be less verbose, use the new 2-argument version of strscpy()
which computes auto-magically the size of the destination.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/bf8a9353319566624f653531b80e5caf3d346ba1.1711176700.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>