serial: Enhance the manual relocation

Enhance the manual relocation of drivers operations structure by
checking if the entries are NULL and increment them only if they
are not. This allows for setting any entry to NULL and it will
survive the manual relocation.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>