RDMA/bnxt_re: improve clarity in ALLOC_PAGE handler

RDMA/bnxt_re: improve clarity in ALLOC_PAGE handler

Update uverbs_copy_to call to use sizeof(dpi) instead of sizeof(length)
when copying the device page index (DPI) back to user space. Both dpi
and length are declared as u32, so this change has no functional impact
but makes the code clearer.

Link: https://patch.msgid.link/r/20250924110130.340195-1-alok.a.tiwari@oracle.com
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>