rsi: Delete unnecessary variable initialisation

In rsi_send_mgmt_pkt(), the following variables are assigned to
before they're used:

 * wh - Assigned on line 161, first used on line 180
 * bss - Assigned on line 160, first used on line 196
 * msg - Assigned on line 168, first used on line 175
 * extnd_size - Assigned on line 139, first used on line 142

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[Rewrote commit message]
Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>