mbedtls: fix _mbedtls_init() invocation

SYS_INIT() requires pointer to function that takes `void` now, instead of
pointer to device structure. Since the commit was developed before that
switch, it still invoked it with NULL. Fix that now.

Fixes:

  zephyr/modules/mbedtls/zephyr_init.c: In function 'mbedtls_init':
  zephyr/modules/mbedtls/zephyr_init.c:108:16: error: too many arguments \
                                              to function '_mbedtls_init'
    108 |         return _mbedtls_init(NULL);
        |                ^~~~~~~~~~~~~
  zephyr/modules/mbedtls/zephyr_init.c:86:12: note: declared here
     86 | static int _mbedtls_init(void)
        |

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>