greybus: drop gfp_mask from gb_message_send()

We will only send messages from process context.  Drop the gfp_mask
parameter from gb_message_send(), and just supply GFP_KERNEL to the
host driver's buffer_send method.

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