common: mmc: unsigned char compared against 0

"enable" is unsigned char type and its value will not be
negative, so discard "enable < 0".

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Diego Santa Cruz <Diego.SantaCruz@spinetix.com>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>