dell-smbios: implement new function for finding DMI table 0xDA tokens

Ultimately, the da_tokens table should not be exported from dell-smbios.
Currently, in some cases, dell-laptop accesses that table's members
directly, so implement a new function, dell_smbios_find_token(), which
returns a pointer to an entry inside the da_tokens table with the given
token ID (or NULL if it is not found).

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>