mwifiex: Do not change bss_num in change_virtual_intf

Commit 4d7ab36f ("mwifiex: Do not change bss_type in
change_virtual_intf") kept original bss_type unchanged. bss_num should
keep the same style, in this way. Unique tuple (bss_type, bss_num) will
be able to locate the right priv structure.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>