dell-smbios: return the SMBIOS buffer from dell_smbios_get_buffer()

Ultimately, the SMBIOS buffer should not be exported from dell-smbios.
Currently, dell-laptop accesses it directly using a global variable, so
make dell_smbios_get_buffer() return a pointer to the SMBIOS buffer and
replace all uses of the global variable with local variables.

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>