fwctl/mlx5: Fix memory alloc/free in mlx5ctl_fw_rpc()

Use kvfree() to free memory allocated by kvzalloc() instead of kfree().
Avoid potential memory management issue considering kvzalloc() can
internally choose to use either kmalloc() or vmalloc() based on memory
request and current system memory state. Hence, use more appropriate
kvfree() which automatically determines correct free method to avoid
potential hard to debug memory issues. Fix this issue discovered by
running spatch static analysis tool using coccinelle script -
scripts/coccinelle/api/kfree_mismatch.cocci

Fixes: 52929c2142041 ("fwctl/mlx5: Support for communicating with mlx5 fw")
Link: https://patch.msgid.link/r/aKAjCoF9cT3VEbSE@bhairav-test.ee.iitb.ac.in
Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>