crypto: inside-secure - Fix a maybe-uninitialized warning

A previous fixup avoided an unused variable warning but replaced
it with a slightly scarier warning:

drivers/crypto/inside-secure/safexcel.c:1100:6: error: variable 'irq' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]

This is harmless as it is impossible to get into this case, but
the compiler has no way of knowing that. Add an explicit error
handling case to make it obvious to both compilers and humans
reading the source.

Fixes: 212ef6f2

 ("crypto: inside-secure - Fix unused variable warning when CONFIG_PCI=n")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Pascal van Leeuwen <pvanleeuwen@verimatrix.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>