spmi: Add a check for remove callback when removing a SPMI driver

In the Linux kernel, the following vulnerability has been resolved:

spmi: Add a check for remove callback when removing a SPMI driver

When removing a SPMI driver, there can be a crash due to NULL pointer

dereference if it does not have a remove callback defined. This is

one such call trace observed when removing the QCOM SPMI PMIC driver:

dump_backtrace.cfi_jt+0x0/0x8

dump_stack_lvl+0xd8/0x16c

panic+0x188/0x498

__cfi_slowpath+0x0/0x214

__cfi_slowpath+0x1dc/0x214

spmi_drv_remove+0x16c/0x1e0

device_release_driver_internal+0x468/0x79c

driver_detach+0x11c/0x1a0

bus_remove_driver+0xc4/0x124

driver_unregister+0x58/0x84

cleanup_module+0x1c/0xc24 [qcom_spmi_pmic]

__do_sys_delete_module+0x3ec/0x53c

__arm64_sys_delete_module+0x18/0x28

el0_svc_common+0xdc/0x294

el0_svc+0x38/0x9c

el0_sync_handler+0x8c/0xf0

el0_sync+0x1b4/0x1c0

If a driver has all its resources allocated through devm_() APIs and

does not need any other explicit cleanup, it would not require a

remove callback to be defined. Hence, add a check for remove callback

presence before calling it when removing a SPMI driver.