power: smb135x-charger: serialize access to registers

There could be instances when multiple threads execute read modify
write on same register leading to nasty race conditions.

Protect the calls to read, write and modify registers.

Change-Id: I4252befdd2e68685f866e0504a74b3281011912d
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>