media: bcm2835-unicam: Set ret on error path in unicam_async_complete()

Clang warns:

  drivers/media/platform/bcm2835/bcm2835-unicam.c:3109:6: warning: variable 'ret' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
          if (!source_pads) {
              ^~~~~~~~~~~~
  drivers/media/platform/bcm2835/bcm2835-unicam.c:3152:9: note: uninitialized use occurs here
          return ret;
                 ^~~
  drivers/media/platform/bcm2835/bcm2835-unicam.c:3109:2: note: remove the 'if' if its condition is always false
          if (!source_pads) {
          ^~~~~~~~~~~~~~~~~~~
  drivers/media/platform/bcm2835/bcm2835-unicam.c:3091:9: note: initialize the variable 'ret' to silence this warning
          int ret;
                 ^
                  = 0
  1 warning generated.

When the if condition is true, ret will be used uninitialized, which
could result in undesirable behavior. Set ret to -ENODEV on the error
path, which is a standard error code for the ->complete() callback.

Fixes: d056e86e ("media/bcm2835-unicam: Parse pad numbers correctly")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>