usb: gadget: function: f_uac1: implement get_alt()

After commit 7e4da3fcf7c9 ("usb: gadget: composite:
Test get_alt() presence instead of set_alt()") f_uac1
function became broken because it doesn't have
get_alt() callback implementation and composite
framework never set altsetting 1 for audiostreaming
interface. On host site it looks like:

 [424339.017711] 21:1:1: usb_set_interface failed (-32)

Since host can't set altsetting 1, it can't start
playing audio.

In order to fix it implemented get_alt along with
minor improvements (error conditions checking)
similar to what existing f_uac2 has.

Change-Id: I0a8371ed74a1b27cbba2261e5dab28c8d4af44b2
Cc: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Git-commit: 1fc4926d92b9515b44f35b339bab5d2ca474a723
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>