bnxt: avoid overflow in bnxt_get_nvram_directory()

bnxt: avoid overflow in bnxt_get_nvram_directory()

The value of an arithmetic expression is subject
of possible overflow due to a failure to cast operands to a larger data
type before performing arithmetic. Used macro for multiplication instead
operator for avoiding overflow.

Found by Security Code and Linux Verification
Center (linuxtesting.org) with SVACE.

Signed-off-by: Maxim Korotkov <korotkov.maxim.s@gmail.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://lore.kernel.org/r/20230309174347.3515-1-korotkov.maxim.s@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>