char: xdevcfg: Check 'sysfs_create_group' return value

Test the return value of 'sysfs_create_group()' and issue a warning if
the call fails. This silences the following build warning:
   drivers/char/xilinx_devcfg.c: In function 'xdevcfg_fclk_init':
	drivers/char/xilinx_devcfg.c:1785:20: warning: ignoring return
	value of 'sysfs_create_group', declared with attribute
	warn_unused_result [-Wunused-result]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>