asm-generic: rwsem: ensure sem->cnt is only accessed via atomic_long_*

The asm-generic rwsem implementation directly acceses sem->cnt when
performing a __down_read_trylock operation. Whilst this is probably safe
on all architectures, we should stick to the atomic_long_* API and use
atomic_long_read instead.

Change-Id: I03cdf1bd7377b9af2374affdbe422fb0ec03cf21
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>