platform/x86: acerhdf: clarify modinfo messages for BIOS override

Normally, a module parameter for a BIOS check override implies "pretend
you support this version" (and the user will enter their local version).

However, this driver uses the model/BIOS module parameters in a way that
is "pretend my system is the supported model XYZ with BIOS version ABC."
which is less common.

Since the help strings don't make such a distinction, one gets this
somewhat frustrating scenario, where the user sees the error, enters
*their* BIOS version and then gets the same error:

  root@gw:~# modprobe acerhdf
  acerhdf: Acer Aspire One Fan driver, v.0.7.0
  acerhdf: unknown (unsupported) BIOS version Gateway  /LT31   /v1.3307 , please report, aborting!
  modprobe: ERROR: could not insert 'acerhdf': Invalid argument

  root@gw:~# modprobe acerhdf force_bios=v1.3307
  acerhdf: Acer Aspire One Fan driver, v.0.7.0
  acerhdf: forcing BIOS version: v1.3307
  acerhdf: unknown (unsupported) BIOS version Gateway  /LT31   /v1.3307 , please report, aborting!
  modprobe: ERROR: could not insert 'acerhdf': Invalid argument

Clarify the module param help text to make it clear that the driver
expects a choice from existing supported models/versions.

Cc: Peter Feuerer <peter@piie.net>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Andy Shevchenko <andy@infradead.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Peter Feuerer <peter@piie.net>