usb/mcs7830: Don't use buffers from stack for USB transfers

mcs7830_set_reg() and mcs7830_get_reg() are called with buffers
from stack which must not be used directly for USB transfers.
This causes corruption of the stack particulary on non x86
architectures because DMA may be used for these transfers.

Signed-off-by: Christian Eggers <christian.eggers@kathrein.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>