x86/resctrl: Merge AMD/Intel parse_bw() calls

Now that we've explained arch_needs_linear to resctrl, the parse_bw()
calls are almost the same between AMD and Intel.

The difference is '!is_mba_sc()', which is not checked on AMD. This
will always be true on AMD CPUs as mba_sc cannot be enabled as
is_mba_linear() is false.

Removing this duplication means user-space visible behaviour and
error messages are not validated or generated in different places.

CC: Babu Moger <Babu.Moger@amd.com>
Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>