drivers/bitbang: silence scan-build warning

drivers/bitbang: silence scan-build warning

The array is partially initialized with buf_set_u32(,5,32,), then
the rest of the array is read from SWD.
But scan-build report the array to have garbage content after the
initialization, due to the offset of 5 bit that only inits part of
the first byte.

Silence the false positive from scan-build by initializing the
array.

Change-Id: Ic38d50280f67939e3ec5fa05741f66d5f993f8c2
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7013
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>