ipmi: ipmb: fix dependencies to eliminate build error

When CONFIG_I2C=m, CONFIG_I2C_SLAVE=y (bool), and CONFIG_IPMI_IPMB=y,
the build fails with:

ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_remove':
ipmi_ipmb.c:(.text+0x6b): undefined reference to `i2c_slave_unregister'
ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_thread':
ipmi_ipmb.c:(.text+0x2a4): undefined reference to `i2c_transfer'
ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_probe':
ipmi_ipmb.c:(.text+0x646): undefined reference to `i2c_slave_register'
ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_driver_init':
ipmi_ipmb.c:(.init.text+0xa): undefined reference to `i2c_register_driver'
ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_driver_exit':
ipmi_ipmb.c:(.exit.text+0x8): undefined reference to `i2c_del_driver'

This is due to having a tristate depending on a bool symbol.
By adding I2C (tristate) as a dependency, the desired dependencies
are met, causing IPMI_IPMB to be changed from =y to =m:

  -CONFIG_IPMI_IPMB=y
  +CONFIG_IPMI_IPMB=m

Fixes: 63c4eb3471 ("ipmi:ipmb: Add initial support for IPMI over IPMB")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Corey Minyard <minyard@acm.org>
Cc: openipmi-developer@lists.sourceforge.net
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Message-Id: <20211012204416.23108-1-rdunlap@infradead.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

drivers/char/ipmi/Kconfig

@@ -77,7 +77,7 @@ config IPMI_SSIF
 
 config IPMI_IPMB
 	tristate 'IPMI IPMB interface'
-	depends on I2C_SLAVE
+	depends on I2C && I2C_SLAVE
 	help
 	  Provides a driver for a system running right on the IPMB bus.
 	  It supports normal system interface messages to a BMC on the IPMB