Fixed Build warning wfd_util_get_access_list

Fixed Build warning wfd_util_get_access_list

Description: This patch removes build warning in
wfd_util_get_access_list, in this funtion integer was compared
to NULL, which is wrong and was generating warning.

As per man pages:
fgets return char* and it's ouput was assigned to an integer

] wifi-direct-util.c: In function 'wfd_util_get_access_list':
] warning: assignment makes integer from pointer without a cast
]   while ((res = fgets(device_info, MACSTR_LEN + DEV_NAME_LEN + 2, fd)) != NULL)
]               ^
] warning: comparison between pointer and integer
]   while ((res = fgets(device_info, MACSTR_LEN + DEV_NAME_LEN + 2, fd)) != NULL)
]                                                                        ^

Change-Id: I7600a44ecc808f15f11649206606313fe614e835
Signed-off-by: Nishant Chaprana <n.chaprana@samsung.com>