staging: vt6655: fix sparse warning for static declarations

This patch fixes the following sparse warnings:

dpc.c:65:21: warning: symbol 'acbyRxRate' was not declared. Should it be static?
dpc.c:272:9: warning: symbol 'MngWorkItem' was not declared. Should it be static?
dpc.c:288:1: warning: symbol 'device_receive_frame' was not declared. Should it be static?

Signed-off-by: James A Shackleford <shack@linux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>