greybus: loopback: rename frequency to requests

The name frequency does not adequately describe the data-point tracking the
number of greybus operations performed in a second by the loopback code.
Firmware team is moving towards calling this variable requests-per-second
or similar. This patch renames to keep the namespace consistent.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>