PRISM54: fix compilation warning

CC [M]  drivers/net/wireless/prism54/islpci_eth.o
drivers/net/wireless/prism54/islpci_eth.c: In function ‘islpci_eth_cleanup_transmit’:
drivers/net/wireless/prism54/islpci_eth.c:53: warning: cast from pointer to integer of different size
drivers/net/wireless/prism54/islpci_eth.c: In function ‘islpci_eth_receive’:
drivers/net/wireless/prism54/islpci_eth.c:453: warning: cast from pointer to integer of different size

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>