misc: amd-sbi: Optimize the wait condition for mailbox command completion

- optimize the wait condition to indicate command completion
  by replacing the do while loop with regmap subsystem API
  regmap_read_poll_timeout()

Reviewed-by: Naveen Krishna Chatradhi <naveenkrishna.chatradhi@amd.com>
Signed-off-by: Akshay Gupta <akshay.gupta@amd.com>
Link: https://lore.kernel.org/r/20250428063034.2145566-6-akshay.gupta@amd.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>