bnxt_en: Convert error code in firmware message response to standard code.

The main firmware messaging function returns the firmware defined error
code and many callers have to convert to standard error code for proper
propagation to userspace.  Convert bnxt_hwrm_do_send_msg() to return
standard error code so we can do away with all the special error code
handling by the many callers.

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