efivarfs: convert to use simple_open()

This removes an open coded simple_open() function and
replaces file operations references to the function
with simple_open() instead.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: Matthew Garret <mjg59@srcf.ucam.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>