thunderbolt: Check return value of tb_sw_read() in usb4_switch_op()

The function misses checking return value of tb_sw_read() before it
accesses the value that was read. Fix this by checking the return value
first.

Fixes: b0407983 ("thunderbolt: Add initial support for USB4")
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Yehezkel Bernat <yehezkelshb@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>