mwifiex: remove unnecessary code in data rate configuration

1) Remove unnecessary wrapper functions.
2) Currently we don't have command to set Tx data rate, so
mwifiex_rate_ioctl_set_rate_value() function and related code
can be removed.
3) "ds_rate" filled by mwifiex_ret_tx_rate_cfg() is never used.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>