zd1211rw: use stack and preallocated memory for small cmd-buffers

Use stack for allocing small < 64 byte arrays in zd_chip.c and preallocated
buffer in zd_usb.c. This might lower CPU usage for beacon setup.

v2:
 - Do not use stack buffers in zd_usb.c as they would be used for urb
   transfer_buffer.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>