media: dvb-frontends: remove redundant words and fix several typos

change 'purpous' to 'purpose'.
change 'frequecy' to 'frequency'.
remove redundant words struct and enum.

Signed-off-by: wengjianfeng <wengjianfeng@yulong.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>