mwifiex: endian bug in mwifiex_drv_change_adhoc_chan()

In mwifiex_drv_change_adhoc_chan() we pass channel to
mwifiex_bss_ioctl_ibss_channel() which sets the high 16 bits.  This
works on little endian systems but not on big endian ones.  I've changed
mwifiex_drv_change_adhoc_chan() to use a 16 bit variable.  This matches
the other functions in the file.

I considered changing mwifiex_change_adhoc_chan() as well but it turns
out that the function isn't implemented so I just removed the
definition.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>