mmc: alcor: remove a redundant greater or equal to zero comparison

A greater or equal comparison on the unsigned int variable tmp_diff
is always true as unsigned ints are never negative.  Hence the
comparison is redundant and can be removed.

Addresses-Coverity: ("Unsigned compared against 0")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>