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.