rtlwifi: remove redundant initialization to cfg_cmd

cfg_cmd is initialized to zero and this value is never read, instead
it is over-written in the start of a do-while loop. Remove the
redundant initialization. Cleans up clang warning:

drivers/net/wireless/realtek/rtlwifi/core.c:1750:22: warning: Value
stored to 'cfg_cmd' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>