fs: exfat: Fix conversion overflow errors

fs: exfat: Fix conversion overflow errors

Fix the following conversion overflow errors. The UTF8-to-UTF16
conversion is done through 32bit wchar_t, but U-Boot codebase is
built with -fshort-wchar which limits wchar_t to 16bit. Replace
the built-in wchar_t with u32 to assure the intermediate type is
32bit.

"
fs/exfat/utf.c: In function ‘utf8_to_wchar’:
fs/exfat/utf.c:165:23: warning: overflow in conversion from ‘int’ to ‘wchar_t’ {aka ‘short unsigned int’} changes value from ‘(int)(short unsigned int)*input << 18 & 1835008’ to ‘0’ [-Woverflow]
  165 |                 *wc = ((wchar_t) input[0] & 0x07) << 18;
      |                       ^
fs/exfat/utf.c:170:23: warning: overflow in conversion from ‘int’ to ‘wchar_t’ {aka ‘short unsigned int’} changes value from ‘(int)(short unsigned int)*input << 24 & 50331648’ to ‘0’ [-Woverflow]
  170 |                 *wc = ((wchar_t) input[0] & 0x03) << 24;
      |                       ^
fs/exfat/utf.c:175:23: warning: overflow in conversion from ‘int’ to ‘wchar_t’ {aka ‘short unsigned int’} changes value from ‘(int)(short unsigned int)*input << 30 & 1073741824’ to ‘0’ [-Woverflow]
  175 |                 *wc = ((wchar_t) input[0] & 0x01) << 30;
      |                       ^
"

Signed-off-by: Marek Vasut <marex@denx.de>