drivers/mmc/fsl_esdhc.c: fix compiler warnings

Commit 77c1458d caused the following compiler warnings:

fsl_esdhc.c: In function 'esdhc_pio_read_write':
fsl_esdhc.c:142: warning: assignment discards qualifiers from pointer target type
fsl_esdhc.c: In function 'esdhc_setup_data':
fsl_esdhc.c:169: warning: unused variable 'wml_value'
fsl_esdhc.c: In function 'esdhc_pio_read_write':
fsl_esdhc.c:164: warning: control reaches end of non-void function

Fix these.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Dipen Dudhat <dipen.dudhat@freescale.com>
Cc: Andy Fleming <afleming@freescale.com>