transport: fix incorrect statement

transport: fix incorrect statement

Commit 236208a5ff2d ("transport: use a bitmask for the transport")
has an incorrect C statement in place of a return.
The code is working thanks to the previous condition never true.

The issue has been detected by clang scan-build in OpenOCD ACI
since the missing return can make the following statement
dereferencing a NULL pointer.

Fix it!

Change-Id: I3bbe04d99ad9b1288f55ba3c45e2e487aef9ae40
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 236208a5ff2d ("transport: use a bitmask for the transport")
Reviewed-on: https://review.openocd.org/c/openocd/+/8868
Tested-by: jenkins