linuxkpi/dmi: don't match exactly on DMI_MATCH

Linux has two defines to check dmi data. DMI_MATCH checks if the dmi
string includes substr. DMI_EXACT_MATCH checks if the dmi string exactly
matches substr. Compat layer should have the same behaviour.

The new definition of dmi_strmatch shouldn't break any driver. A driver
would break if it uses the highest bit of the slot field. Nevertheless,
linux uses the same definition and FreeBSD uses dmi_field values as slot
which are lower than 128.

Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35395