fat: correct ATTR_VFAT check

ATTR_VFAT condition requires multiple bits to be set but the present
condition checking in do_fat_read() & get_dentfromdir() ends up
passing on even a single bit being set.

Signed-off-by: J. Vijayanand <vijayanand.jayaraman@in.bosch.com>
Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>