bnxt_en: Remove unneeded MSIX base structure fields and code

Ever since commit:

30343221 ("bnxt_en: Remove runtime interrupt vector allocation")

The MSIX base vector is effectively always 0.  Remove all unneeded
structure fields and code referencing the MSIX base.

Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20240409215431.41424-4-michael.chan@broadcom.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>