staging: dgnc: Fix frame size is larger than 1024B

This comit fixes the following sparse warnign:

drivers/staging/dgnc/dgnc_tty.c:572:1:
    warning: the frame size of 1060 bytes is larger than 1024 bytes
    [-Wframe-larger-than=]

This was caused by having buffer as an automatic variable. This commit
moves it from the stack to the heap.

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>