greybus: gb-audio: Allocate space for 20 configurations

The greybus code requires that an operation's response buffer be
the exact size of the response; however, the size of the response to
the GB_I2S_MGMT_TYPE_GET_SUPPORTED_CONFIGURATIONS operation is unknown.
To fix this, an extension to the I2S specification is required.

In the meantime, set the number of configurations returned to 20
because that is how many configurations will be returned (using
"insider knowledge" of the firmware).

Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>