ata: libata: Add support to parse equal sign in libata.force

ata: libata: Add support to parse equal sign in libata.force

Currently, no libata.force parameter supports an arbitrary value.

All allowed values, e.g. udma/16, udma/25, udma/33, udma/44, udma/66,
udma/100, udma/133 have hardcoded entries in the force_tbl table.

Add code to allow a libata.force param with the format
libata.force=param=param_value, where param_value can be an arbitrary
value.

This code will be used in a follow up commit.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>