staging: wlags49_h2: fix error return code in wl_adapter_insert()

When return from wl_adapter_insert() in the register_netdev() error
handling case, 'ret' which is 0 is returned, but we should return a
negative error code instead, so fix to return the return value of
register_netdev().

Introduce by commit 657d4c86
(staging: wlags49_h2: fix error handling in pcmcia probe function)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>