drivers: wifi: simplelink: Fix error handling

The driver is not returning correct error codes when error occurs.
The error handling function is expecting negative input values, but
that is not true for BSD error codes. So I am taking an approach
where I use a function to convert SimpleLink error codes to BSD
error codes, and call slcb_setErrno() to set the errno independently.

Fixes #12745

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>