power: qpnp-fg: fix a potential access lockup

In fg_mem_read() there is an execution path where releasing access to
the fuel gauge sram may fail but the associated rw_lock mutex is not
released.

Fix the potential deadlock.

Change-Id: I08edb83135203dd37114b4bed6aa580defc04210
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>