iwl3945: use hw_params.rx_buf_size

This patch makes 3945 use of hw_params.rx_buf_size instead of
IWL_RX_BUF_SIZE. It also renames IWL_RX_BUF_SIZE to IWL_RX_BUF_SIZE_3K
and moves rx buffer defines into iwl-fh.h.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>