b43: silence a bogus gcc warning

use uninitialized_var() to avoid the following bogus warning:
  CC [M]  drivers/net/wireless/b43/debugfs.o
drivers/net/wireless/b43/debugfs.c: In function ‘b43_debugfs_read’:
drivers/net/wireless/b43/debugfs.c:355: warning: ‘ret’ may be used uninitialized in this function

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>