r92su: fix errors found by smatch

main.c:218 r92su_find_wmm_ie() info: loop could be replaced with if statement.
main.c:202 r92su_find_wmm_ie() info: ignoring unreachable code.
main.c:656 r92su_bss_connect_work() warn: variable dereferenced before check 'cfg_bss' (see line 653)
main.c:1106 r92su_bss_build_fw_bss() error: we previously assumed 'tmp' could be null (see line 1106)

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>