ACPI: fix allowing to add/remove multiple _OSI strings

commit b0ed7a91(ACPICA/ACPI: Add new host interfaces for _OSI suppor)
introduced another regression that only one _OSI string can be added or
removed.

Now multiple _OSI strings can be added or removed, for example

acpi_osi=Linux acpi_osi=FreeBSD acpi_osi="!Windows 2006"

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>