canmb board: Fix compiler warnings

Recent changes caused thatthe aev board now is included in the boards
built by MAKEALL, which revealed that compilation for this board
has been broken for a long time:

canmb.c: In function 'initdram':
canmb.c:109: warning: pointer targets in passing argument 1 of 'get_ram_size' differ in signedness
canmb.c:111: warning: pointer targets in passing argument 1 of 'get_ram_size' differ in signedness
canmb.c:137: warning: pointer targets in passing argument 1 of 'get_ram_size' differ in signedness
canmb.c:140: warning: pointer targets in passing argument 1 of 'get_ram_size' differ in signedness

Fix these.

Signed-off-by: Wolfgang Denk <wd@denx.de>