struct mmc should be clear to all '0' after malloc to avoid
unexpect variable value.

Like mmc->has_init = xxx.
In this case mmcinfo will believe the card had been initialized before
and skip the initialization.

Test on P5040 and T4240,
Error Log:

=> mmcinfo
Device: FSL_SDHC
Manufacturer ID: 0
OEM: 0
Name: Tran Speed: 0
Rd Block Len: 0
MMC version 0.0
High Capacity: No
Capacity: 0 Bytes
Bus Width: 0-bit
=>
Signed-off-by: NHaijun Zhang <Haijun.Zhang@freescale.com>
Signed-off-by: NXie Shaohui-B21989 <B21989@freescale.com>
Tested-by: NRyan Barnett <rjbarnet@rockwellcollins.com>
Acked-by: NPantelis Antoniou <panto@antoniou-consulting.com>