devlink: fix uninitialized warning

devlink: fix uninitialized warning

GCC-10 complains about uninitialized variable.

devlink.c: In function ‘cmd_dev’:
devlink.c:2803:12: warning: ‘val_u32’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 2803 |    val_u16 = val_u32;
      |    ~~~~~~~~^~~~~~~~~
devlink.c:2747:11: note: ‘val_u32’ was declared here
 2747 |  uint32_t val_u32;
      |           ^~~~~~~

This is a false positive because it can't figure out the control flow
when the parse returns error.

Fixes: 2557dca2b028 ("devlink: Add string to uint{8,16,32} conversion for generic parameters")
Cc: shalomt@mellanox.com
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>