ASoC: Intel: mrfld: simplify sst_fill_widget_module_info

Some tiny improvements, cutting 180 bytes off the generated code.

- use strchr() for single-character needle

- compute index using pointer subtraction instead of two strlen()
  calls

- factor out the common check for whether the initial part of
  kctl->id.name (before the space) is identical to w->name.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Tested-by: Fang, Yang A <yang.a.fang@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>