s390/raw3270: add raw3270_start_request() helper

There are a few places (and there would be more with the following commits)
like this:

raw3270_request_reset(cp->kreset);
raw3270_request_set_cmd(cp->kreset, TC_WRITE);
raw3270_request_add_data(cp->kreset, &kreset_data, 1);
raw3270_start(&cp->view, cp->kreset);

i.e reset a request, setting the command, adding payload, and starting the
request. Add a helper raw3270_start_request() which takes a command and
the payload as argument and calls the approppriate functions.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>