common: mmc: unsigned char compared against 0

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 <[email protected]>
Cc: Diego Santa Cruz <[email protected]>
Cc: Pantelis Antoniou <[email protected]>
Cc: Andrew Gabbasov <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>