bnxt: avoid overflow in bnxt_get_nvram_directory()

In the Linux kernel, the following vulnerability has been resolved:

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.