RDMA/erdma: Eliminate unnecessary casting of EQ doorbells

Using void * to define EQ doorbell pointer can eliminate unnecessary
casting when performing assignment. Also rename *db_addr* to *db* for a
shorter name.

Signed-off-by: Cheng Xu <chengyou@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230322093319.84045-3-chengyou@linux.alibaba.com


Signed-off-by: Leon Romanovsky <leon@kernel.org>