bnxt_en: Avoid arithmetic on void * pointer.

Typecast hwrm_cmd_resp_addr to (u8 *) from (void *) before doing
arithmetic.

Signed-off-by: Venkat Duvvuru <venkatkumar.duvvuru@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>