bnxt_en: refactor bnxt_alloc_fw_health()

The allocator for the firmware health structure conflates allocation
and capability checks, limiting the reusability of the code. This patch
separates out the capability check and disablement and improves the
warning message to better describe the consequences of an allocation
failure.

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>