msm: smd_pkt: Register the platform driver while performing open operation

Defer registering the platform driver for an SMD channel until the SMD_PKT
driver open operation is invoked.

Previously during initializaton, SMD_PKT driver would register a platform
driver for all SMD channels in the configuration table. This prevented
other drivers from being able to register for the same SMD channel, even
if the SMD_PKT driver never uses that channel.

Change-Id: Ie9b80329093c0b14bf0f74280fb30bdd3d3aa7a9
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>