sdhci: be more strict with get_min_clock() usage

sdhci: be more strict with get_min_clock() usage

get_min_clock() makes sense only with NONSTANDARD_CLOCK quirk and when
set_clock() callback is specified.

The patch should cause no functional changes, it just makes the code
self-documented and avoids any possible misuse of get_min_clock().

Suggested-by: Pierre Ossman <pierre@ossman.eu>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Ian Molton <ian@mnementh.co.uk>
Cc: Matt Fleming <matt@console-pimps.org>
Cc: Philip Langdale <philipl@overt.org>
Cc: Pierre Ossman <pierre@ossman.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>