mmc_da: make sure that part_index is not used uninitialized in sddastart

This is a fix to r334065.

Without this change I once got stuck I/O with endless partition switching:

(sdda0:aw_mmc_sim2:0:0:0): sddastart
(sdda0:aw_mmc_sim2:0:0:0): Partition 0 -> -525703168
(sdda0:aw_mmc_sim2:0:0:0): xpt_action: func 0x91d XPT_MMC_IO
(sdda0:aw_mmc_sim2:0:0:0): xpt_done: func= 0x91d XPT_MMC_IO status 0x1
(sdda0:aw_mmc_sim2:0:0:0): sddadone
(sdda0:aw_mmc_sim2:0:0:0): Card status: 00000000
(sdda0:aw_mmc_sim2:0:0:0): Current state: 4
(sdda0:aw_mmc_sim2:0:0:0): Compteting partition switch to 0

Note that -525703168 (an int) is 0xe0aa6800 in binary representation.
The partition indexes are actually stored as uint8_t, so that value
was converted / truncated to zero.

MFC after: 1 week