usb: pd: Prevent race between handle_vdm_tx & reset_vdm_state

When type-C disconnects and hard reset happens in parallel,
a race condition is seen between handle_vdm_tx and
reset_vdm_state. This race condition leads to slab poisoning.
To fix this, execute both the functions serially using
mutex locks.

Change-Id: Ie56331e2462e47651a2f170eae0fb8f1655c32f4
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>