ARM: rpc: check device_register return code in ecard_probe

device_register is marked __must_check, so we better propagate the error
value by returning it from ecard_probe.

Without this patch, building rpc_defconfig results in:

arch/arm/mach-rpc/ecard.c: In function 'ecard_probe':
arch/arm/mach-rpc/ecard.c:963:17: warning: ignoring return value of 'device_register', declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>