gianfar: Fix possible overrun and simplify interrupt name field creation

Space allocated for int_name_<foo> is insufficient for
maximal device name, expand it.

Code to create int_name_<foo> is obscure, simplify it
by using sprintf.

Found by looking for unnecessary \ line continuations.

Signed-off-by: Joe Perches <joe@perches.com>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>