staging: rtl8712: r8712_joinbss_cmd(): Change return values and type

Change return values of function r8712_joinbss_cmd from _SUCCESS/_FAIL
to 0/-ENOMEM respectively.
Change return type from u8 to int to accommodate return of -ENOMEM.

Similarly, change the return values (_SUCCESS to 0 and _FAIL to -ENOMEM
or -EINVAL) and type (sint to int) of the call site of r8712_joinbss_cmd,
r8712_select_and_join_from_scan, as one of the branches of
r8712_select_and_join_from_scan directly returns the return value of
r8712_joinbss_cmd.

Modify the call sites of r8712_select_and_join_from_scan to check for
0 instead of _SUCCESS.

Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>