gatt: actually drop lock when calling read callback

Same as in all other cases, GATT should drop the GATT lock before
calling user callback. Read code did not do this..until now

BUG=b:74813838
TEST=no longer deadlocks when read callback requests another read right from the calback
Change-Id: I2433672b1b54cf987b208f06d88793c933b68fe6
1 file changed