driver/mmc: fix compile warnings

Fix following compile warnings
fsl_esdhc_spl.c: In function 'mmc_boot':
fsl_esdhc_spl.c:35:10: warning: unused variable 'byte_num' [-Wunused-variable]
fsl_esdhc_spl.c:35:7: warning: unused variable 'i' [-Wunused-variable]
fsl_esdhc_spl.c:34:8: warning: unused variable 'val' [-Wunused-variable]
fsl_esdhc_spl.c:33:6: warning: unused variable 'blklen' [-Wunused-variable]
fsl_esdhc_spl.c:105:7: warning: 'tmp_buf' may be used uninitialized in this
function [-Wuninitialized]

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>