ENGR00259008: mlb: reduce iram usage amount in async mode

ENGR00259008: mlb: reduce iram usage amount in async mode

In testing async mode on mx6q ard and mx6dl ard, driver always said "can
not alloc rx buffer".
Change async's ring buffer size from 2048 to 1536(MEP package size) and
reduce the extra ring buffer for drop package, now the iram usage amount
in async mode reduced from 34816 to 24576.

Signed-off-by: Terry Lv <r65388@freescale.com>