rpmsg: Use strscpy() instead of strscpy_pad()

rpmsg: Use strscpy() instead of strscpy_pad()

kzalloc() already zero-initializes the destination buffer, making
strscpy() sufficient for safely copying the name. The additional NUL-
padding performed by strscpy_pad() is unnecessary.

The size parameter is optional, and strscpy() automatically determines
the size of the destination buffer using sizeof() when the argument is
omitted. RPMSG_NAME_SIZE is equal to sizeof(rpdev->id.name) and can be
removed - remove it.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20250429104543.66927-2-thorsten.blum@linux.dev
Signed-off-by: Bjorn Andersson <andersson@kernel.org>