exfat: remove unnecessary reassignment of p_uniname->name_len

kbuild test robot reported :

	fs/exfat/nls.c:531:22: warning: Variable 'p_uniname->name_len'
	is reassigned a value before the old one has been used.

The reassignment of p_uniname->name_len is not needed and remove it.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>