thunderbolt: Rename and make nvm_read() available for other files

In order to support non-Intel NVM formats the vendor specific NVM
validation code that will live in nvm.c needs to be able to read various
parts of the NVM so make the function available outside of switch.c and
rename it accordingly.

Signed-off-by: Szuying Chen <Chloe_Chen@asmedia.com.tw>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>