mmc: block: Avoid uninitialized warning in mmc_blk_issue_discard_rq()

With gcc-4.1.2:

    mmc/core/block.c: In function ‘mmc_blk_issue_discard_rq’:
    mmc/core/block.c:1150: warning: ‘arg’ may be used uninitialized in this function
    mmc/core/block.c:1150: warning: ‘nr’ may be used uninitialized in this function
    mmc/core/block.c:1150: warning: ‘from’ may be used uninitialized in this function

While this is a false positive, it can be avoided easily by jumping over
the checks for "err" that are always false.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>