b43legacy: fix unitialized reads of ret by initializing the array to zero

The u8 char array ret is not being initialized and elements outside
the range start to end contain just garbage values from the stack.
This results in a later scan of the array to read potentially
uninitialized values.  Fix this by initializing the array to zero.
This seems to have been an issue since the very first commit.

Detected by CoverityScan CID#139653 ("Uninitialized scalar variable")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Michael Buesch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>