staging: mt7621-mmc: Fix incompletely removed #if 0 block in sd.c

Commit 2a54e325 ("staging: mt7621-mmc: Remove #if 0 blocks in sd.c")
does not completely remove an #if 0 block in sd.c. This causes the function
msdc_select_clksrc() which was eariler not compiled, to be compiled.
That causes an error - MSDC_CLKSRC_REG is not defined.

This patch completely removes the #if 0 block

Reported-by: NeilBrown <neil@brown.name>
Fixes: 2a54e325 ("staging: mt7621-mmc: Remove #if 0 blocks in sd.c")
Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>