input: misc: qcom-hv-haptics: check nvmem before using it

Currently, nvmem device is used to store closed loop brake settings. If
nvmem-cells is not specified for closed loop brake, then nvmem device
can be NULL. For such case, skip updating closed loop brake settings by
not writing to the nvmem device.

Change-Id: I18d455814f3731af519949f21a3a553bd1692548
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>