greybus: es?: decrease buffer size to 2k

The firmware is having a hard time with 4k buffers and memory
allocation, so decrease the size on the host side to 2k.  Also move away
from using PAGE_SIZE to denote 4k as that's not the case on all
architectures, and someone, someday, might get a rude surprise.

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>