mbr: remove compiler warnings

mbr: remove compiler warnings

mbr.c: In function 'get_ebr_table':
mbr.c:140: warning: implicit declaration of function 'mmc_read_blocks'
mbr.c:133: warning: unused variable 'cylinder'
mbr.c:133: warning: unused variable 'sector'
mbr.c:132: warning: unused variable 'msg'
mbr.c:353: warning: initialization from incompatible pointer type

cmd_usbd.c: In function 'set_mbr_info':
cmd_usbd.c:726: warning: implicit declaration of function 'set_mbr_table'
cmd_usbd.c: In function 'do_usbd_down':
cmd_usbd.c:1401: warning: implicit declaration of function 'get_mbr_table'

Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>