scsi: ufs: Add check before calling device_remove_file

This change adds check if clock scaling is supported before
calling device_remove_file on ufs clk_scaling.enable_attr to
avoid null pointer dereference.

Change-Id: I3bf228c08b5dcf6528f606d0dfa792d10ce408d2
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>