bnxt_en: update all firmware calls to use the new APIs

The conversion follows this general pattern for most of the calls:

1. The input message is changed from a stack variable initialized
using bnxt_hwrm_cmd_hdr_init() to a pointer allocated and intialized
using hwrm_req_init().

2. If we don't need to read the firmware response, the hwrm_send_message()
call is replaced with hwrm_req_send().

3. If we need to read the firmware response, the mutex lock is replaced
by hwrm_req_hold() to hold the response.  When the response is read, the
mutex unlock is replaced by hwrm_req_drop().

If additional DMA buffers are needed for firmware response data, the
hwrm_req_dma_slice() is used instead of calling dma_alloc_coherent().

Some minor refactoring is also done while doing these conversions.

v2: Fix unintialized variable warnings in __bnxt_hwrm_get_tx_rings()
and bnxt_approve_mac()

Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>