if_bnxt: add support for HW generic stats

[sumit.saxena_broadcom.com]

Summary

HW generic stats provide information about PCIe transactions and
out-of-order doorbell writes, useful for debug. Allocate a DMA buffer
for them (disabling BNXT_FW_CAP_GENERIC_STATS if the allocation
fails), fetch them via HWRM_STAT_GENERIC_QSTATS once per admin-status
tick on P5+ chips, and expose them to userspace under
dev.bnxt.<unit>.hwstats.generic_stats.* via sysctl.

[gallatin]

Nice! These stats look really interesting.