ALSA: hdspm - Introduce hdspm_external_rate_to_enum() helper function

This patch refactors the code to query the external sample rate and its
translation into the corresponding enum into a helper function to
prevent future code duplication.

A later commit will make use of this new helper function.

Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>