greybus: get rid of uart request_operation()

In "uart-gb.c", request_operation() function is only used by
get_version().  Since it's not reused, it probably subtracts
rather than adds value.  So just incorporate what it does
into get_version() and get rid of request_operation().

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