greybus: uart: s/REQ/TYPE

Request type for all other protocols is defined like: GB_<protocol>_TYPE_<operation>,
but for UART is like: GB_<protocol>_REQ_<operation>.

Replace REQ with TYPE to make it consistent. It will also be useful in a later
patch that creates get_version() routines with the help of a macro.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>