bnxt_en: add an nvm test for hw diagnose

Add an NVM test function for devlink hw reporter.
In this function an NVM VPD area is read followed by
a write. Test result is cached and if it is successful then
the next test can be conducted only after HW_RETEST_MIN_TIME to
avoid frequent writes to the NVM.

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