pwm: pwm-qti-lpg: Handle nvmem device lookup failure properly

Due to commit 95b65195 ("nvmem: core: Always reference the device
returned by nvmem_device_get()") failures in nvmem device lookup return
-EPROBE_DEFER instead of NULL. This causes probe failure of the
pwm-qti-lpg driver when pwm devices are defined in the device tree
without nvmem nodes, which is undesirable.

Fix this by attempting to get the nvmem device only if it is present.

Change-Id: Ifa136bb9b78e4b9e0894854768e9a6cc060d5261
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>