staging: vt6656: Check the return values in vnt_set_bss_mode function

staging: vt6656: Check the return values in vnt_set_bss_mode function

Check the return value of all the functions that return zero if
successful or a negative error code on failure inside the function
vnt_set_bss_mode.

Also, remove the unnecessary variable initialization as this variable is
set a few lines later.

Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Link: https://lore.kernel.org/r/20200429152307.5871-2-oscar.carter@gmx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>