staging: rtl8723bs: fix a potential memory leak in rtw_init_cmd_priv()

In the Linux kernel, the following vulnerability has been resolved:

staging: rtl8723bs: fix a potential memory leak in rtw_init_cmd_priv()

In rtw_init_cmd_priv(), if `pcmdpriv->rsp_allocated_buf` is allocated
in failure, then `pcmdpriv->cmd_allocated_buf` will be not properly
released. Besides, considering there are only two error paths and the
first one can directly return, so we do not need implicitly jump to the
`exit` tag to execute the error handler.

So this patch added `kfree(pcmdpriv->cmd_allocated_buf);` on the error
path to release the resource and simplified the return logic of
rtw_init_cmd_priv(). As there is no proper device to test with, no runtime
testing was performed.