power: smb5-lib: Fix race conditions for typec power role

Currently power_role is accessed by set_prop and get_prop
functions of typec_power_role without any locking mechanism,
there can be a scenario where both function calls were invoked
simultaneously and power_role variable is not correctly
updated, and it leads to enumeration issues when connected to PC.

Fix it by adding locking mechanism to power_role.

Change-Id: I4f5dc38a9536b535510dc2b112712a5cbd2b3f84
Signed-off-by: Kavya Nunna <knunna@codeaurora.org>