bnxt_en: refactor bnxt_cancel_reservations()

Introduce bnxt_clear_reservations() to clear the reserved attributes only.
This will be used in the next patch to fix PCI AER handling.

Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>